Some ideas (note that a LOT of this would be applicable to ingame images like engravings or drawings as well, even drawings/pictures inside books!):
Shifting writings
One book I read as a child which I can't remember the name of, had an evil wizard with a book full of spells, but it contained more spells than fit on the pages. Rather than the "normal" solution of having infinite pages, the contents changed regularly (cycling through all its "true contents").
That sounds like a neat idea (unlike infinite pages, you can even apply it to other written mediums, like slabs, scrolls, or perhaps tattoos!) and inspiration for some others:
- Writings of any kind (books/slabs) may have more content than actually physically fits on them/can be read by normal means.
- These writings (as in, the physical contents, readable by people) may be changed by deliberate intentional magics/actions looking for specific information.
- May instead be changed automatically according to some criteria (maybe just a schedule/constant, or maybe you need a secret code/word to do it).
- May cycle randomly, or according to the order the information was actually written in, or may "jump" to specific pieces of information, like a wizard "searching" for a specific spell using a magic password, or maybe the book is sentient and you can just ask it for the secrets of life and death, pretty please?
- Instead of having so much content that it just gets full and you need magic to read the rest, some things may look pretty empty, but simply have had the information "hidden" with magic, needing to be revealed by secret actions/passwords, or by waiting for a special schedule. Maybe that slab has nothing but life and death secrets, but you can only read them during the full moon (or under the light of the full moon), or maybe it also has secrets of fire, and for those you need the new moon, or pour some ashes on top of it.
- Some pages may be relatively "fixed" in place, often the first or last pages, which may have unchanging info like the authors name, or some ominous poetic warnings or instructions or whatever. For the likes of scrolls, this comes into play mostly as just specific pieces of information always being visible even while other contents are free to come and go.
Forgettable
Also from the same book mentioned above, it was impossible to use the spells for long after they "left" the books pages, because you would gradually forget them.
So:
- Some magic may cause creatures who have learned some information to forget it; Urist McNecromancer better not lose that slab he'll find out one day he's forgotten the secrets of life and death! Or perhaps someone may send letters containing commands for a minion to carry out, but ensure that the minion can't be interrogated because they'll have forgotten the name the letters (which they burned) were signed with (they'd still probably remember what they were commanded to do, if they had done it, because the magic only affects memory of the info, not what you do with the info; if they were just passing it on to someone else they would probably forget what they said though).
- May be tied to specific sources of information (like a special book or scroll or slab), or may be tied to the actual info itself (maybe Urist McWordWizard wants to make sure the name of Urist McKingWhoUnderpaysWizards is forgotten by all, or maybe the appearance of a special kind of monster is impossible to recall).
- If tied to the source of info, depending on mythgen/world specifics, it may still "contaminate" that info in a sense, so if you try to make a copy of the info, that copy may also possess the same "forgettable" properties. However, it will never make you forget the info if you already knew it (so you can't just write "Urist McDagger killed Urist McVictim" on magic paper and show it to the witnesses), or if you independently learn it from other non-magic affected sources.
- If tied to the info itself, depending on the specifics of the world and its "metaphysics", it may or may not affect people who already knew it before the magic was put into effect on that information (so you might very well make everyone forget that rain exists until it falls on them again, and then they'll forget once more shortly after it stops).
- The speed of forgetting should be taken into account by AI deciding what to do with the magic; instant forgetting is pretty useless if you want someone to act on a command and then forget crucial details about it for example.
- The forgetting may not affect certain creatures; maybe in one world, the author always remembers what they wrote, or more commonly, the gods are above this kind of magic.
No copyright infringement!
Once again (last time I promise!), with same magic book above, you'd think you could get around the memory loss by just writing down the desired spells on some paper and keeping them with you, but nope! Those would fade away, and copies of those copies faded away even faster! Ultimately putting a hard limit on how long you could keep using a spell after it had left the book.
So:
- Prevent certain information from being written down, by making copies inevitably be destroyed (copies of copies of copies etc may or may not be lost even faster than original copies; this should vary depending on the world/magic).
- Copies may be destroyed by normal fading of words, or by the words morphing/becoming illegible or gibberish, or by more unique means related to the magic that's making the copies impossible; for example, if the magic is related to fire, maybe the words catch fire and the medium is destroyed (this could leave some fun "loopholes" for the magic, like writing on fireproof surfaces).
- More overtly, some magics could stop copies being made AT ALL, even temporarily (basically the same thing as fading instantly or becoming illegible/gibberish).
- Whether fading or preventing the creation of copies, this may apply to information from a specific source (like a special magic book), or to a specific piece of information itself; you could imagine Urist McWordWizard trying to wipe the king from history even further by making it impossible to write down their name.
- If applied to the information, it may or may not apply retroactively to information written down already. Some magics might counter this and preserve specific copies though.
Reflecting reality
Some books/mediums might contain information not written down in it by normal means, or may otherwise reflect reality. Usually these would be made by powerful wizards or even gods, or might even be primordial sources of the world itself!:
- All of history ever to occur.
- Everything ever written anywhere else. Might sometimes only contain all current writings, so that if all copies of a book are destroyed, the contents may vanish from this book/whatever as well.
- All names of everybody who ever lived, or perhaps only those who are alive right now. May or may not include "true names".
- The future, unchangeable and fixed in place (would operate similarly to prophecies in general; ie "rigging the RNG to force predicted events to occur"). The scope of this future viewing may vary (some may be scrolls of fate written by the gods that show everything, other times it just predicts what will happen for the next year).
- When containing the future (or past I guess), it may only pertain to certain kinds of events, like deaths and their causes, or all wars (as macroscopic events), or the afterlives of some group (maybe everybody, or maybe just a specific species, or a specific bloodline), or artifacts that will be created.
- Thoughts of creatures/certain people (maybe the god of "truth" has their mind contained in a book and everything they think is laid bare with full honesty).
- Other attributes of individuals.
- May contain information about the world itself (not necessarily history), like its geography, or the species that live in it, or the locations of various resources.
Dictating reality
- If a medium contains the future, if you can write in it you may be able to "change" fate (depending on the magic metaphysics of that medium; some things you can write in and you'd just mess up the book).
- May be blank at the start, but whatever you write in it will come to pass.
- There may be restrictions to what you can write/what kinds of effects you can cause to happen. Maybe immortality or changing people's species is beyond the powers of this magic scroll.
- If the books are selective about information as described in the previous section ("reflecting reality"), naturally only those kinds of things will be alterable; if the book only contains people's thoughts, you obviously won't be able to change the weather using it, but you may be able to force the relevant individuals to think certain things.
- Blank, but if you write the identity of a thing, some effect will be applied to them. Maybe writing their name in the notebook gives them a heart attack.
- Writing down a thing may cause such an object to be manifested, like writing "pot of gold" and getting a pot of gold. May or may not be able to be used to retrieve specific already existing objects, depending on the magic metaphysics (maybe you can only create copies).
- Related to the previous point, some sorts of thing may not be possible to manifest; for example, most of the time you probably wouldn't be able to write new gods into existence, or maybe in this world you can't create anything with magical effects, only mundane objects. Or maybe you can't make living things, or edible things....
- Destroying/erasing parts of the written medium/tearing out book pages may cause objects created using it to vanish (if there is no ontological inertia), or living things to die (depending on the rules for ontological inertia of living things; maybe they just fade too, or turn to ashes).
- Destruction of writings dictating the future (or specific magical effects) may make the future "free" and make it so those things are no longer going to happen.
- Destroying writings dictating special magical effects (like immortality) likely will just "un-apply" those effects. Depending on the laws of this magic in a given world, there may be some retroactive "make it as if it never happened" stuff going on, like instant rapid aging for example, or starving to death immediately if someone was written to not need food.
Magically linked
- Related to the last 2 sections, some AI ingame (or indeed the player) may communicate across distances by use of written media that are "linked", so that writing in one book fills in the other (kind of like texting in some ways really).
- Doesn't necessarily need to be the same kind of medium, though it probably should be usually.
- May be able to have books that magically autofill across distances based on speech; by this I mean, evil demon lord speaks commands from the underworld, and the slab on the surface relays them to his minions. Usually such a slab would probably empty itself after some time/when the demon lord has new commands.
Looks different to everyone
- Very basic idea, some mediums/books may appear to have different contents to different individuals, or different groups of individuals.
- The contents may pertain to the reader personally (like a magical automatic biography).
- The contents may just be a matter of the creator of the medium wanting some information to be hidden from others; maybe their book of secret cooking recipes is disguised as a book about ancient arcane magics. Note this is different to the idea far above about "changing contents", because this way, nobody can shoulder-surf over the wizard for example.
Enforced secrecy
- Readers of a book may be magically prevented from disclosing the contents to anyone else.
- This may be through just forcing them to be unable to speak, or turning their communications into gibberish (or maybe just something else), or making others unable to hear/understand them.
- May often be tied in with preventing the creation of copies.
- Some may be exceptions to this magic (most often this would be the original authors).
Cursed reading
- Reading info may inflict other miscellaneous magical effects on the reader.
- Effects may, depending on the world/magic, only work if the person understands what they are reading (so if you can't read, you might not need to fear being turned into a toad just from looking at it, or maybe when reading skills beyond novice make a difference, unskilled readers may not understand what they're looking at).
- Stackable effects may stack with multiple readings, or by progress into the book (like, with each page/line read). Or maybe not (world-to-world variety!).
- The effects may be based on either the actual reading of the medium itself, or of the information. The difference here is that if it's based off the information, maybe reading about Urist McWizard's secret dwarfy patty formula will turn you into cooking ingredients, but if it's based off the book itself, even if someone erases the contents and writes about something else, it will still transform you.
Don't open it!
- Effects may be applied by merely opening a book/scroll, not even reading it (no real equivalent for slabs I guess).
- These effects may be applied not necessarily to the reader, but to other groups or the world itself, like cursed tomes that unleash chaos or something.
- Books/scrolls like this may tend to have ominous visible seals on them (not necessarily magical or even hard to open, though the more dangerous the effects the harder it would probably be to open, unless the creator of the book actually wanted the bad things to happen).
- Okay, sometimes the effects might be good things, though they may still tend to have seals on them.
Sentience
- Sentient books, scrolls, or slabs/signs etc may be able to communicate, either by audible speech, or by changing the writing magically.
- The may be able to hear normal speech, or it may be impossible for them and they might need to be written in to receive information (they will likely be able to erase the words after they are written, so that they can't get filled up forever and never speak or hear again).
Portals
Written media may be able to act as portals to other dimensions of all kinds (full worlds, pocket dimensions, weird "limbos" where nothing happens and it's just a storage mechanism pretty much etc), and people entering them should be reflected in the text of the medium somehow or another (like, DF doesn't actually generate full texts usually, but mentioning in some natural-seeming way that it contains X character in the world seems like a good idea).
Often, dimensions created by/within books should be alterable by altering the book itself, and may be destroyable (this depends on the exact magical/metaphysical relationship between the book and the dimension though).
Pages
For purposes of magic, it usually shouldn't be the case the vast majority of the time, but separate pages should in principle be able to have different magical properties to one another, even in the same book.