Now that we have books, secrets can still be find on slabs? There other places people can learn secrets?
Secrets can probably still be found on slabs. The books are actually written by the necromancers and seem to document events or people.
Now that writing is in, will dwarves who enter fey moods also do writings if they have the proper skills, thus becoming legendary poets?
If writing is a moodable skill it will probably happen. I think Toady said that literacy was not a skill that could be picked up in fortress mode. I don't think Toady ever mentioned updating moodable skills but I could be wrong.
Well, if that ever happens, I'm so starting a "Unicorns are Hypocrite Bastards" campaign.
If you do something bad to a Good creature, they will attack just as though they were any other creature. The alignment tag does not effect the method of self-defence, only [LIKES_FIGHTING] or [FLEEQUICK] or [PRONE_TO_RAGE:9001] deal with that.
Do apprentices learn the secrets directly from their masters or by using books/slabs found on their towers?
I think either way is fair, but Toady will need to confirm. It may just be that there will be a "has learned secrets of eternal life" flag that just gets checked when the apprentice signs up, abstracting the method out.
Only necromancers can have apprentices or the system will also allow other historical figures to have apprentices?
There may be equivalents to apprentices for the other night creatures, like a master vampire having a number of subordinate lesser vampires. I suspect this may not happen with werewolves. Positions in entities already allow historical figures to have some kind of apprentice, though Toady could add in something where a historical figure (i.e. adventurer or explorer) can take on apprentices.
There will be some kind of Good magic or interaction added at some point, be it healing or something that causes damage to the undead.
According to the latest devblog, the layout of towers were generated on arriving in an area, rather than during worldgen. Is this true of all worldgen locations?