So written stuff is books and could include scrolls, but despite already being used, not much reference was made for slabs, and engravings haven't come up at all.
Do you suppose that literate dwarves might make and enjoy text-oriented engravings? These do seem to be things that are only prevalent in more modern times with widespread literacy but the Egyptians were also known for it, and it would be a good way to get a lot of book content into the fortress as well.
Logically, the most historical cases would include writing alongside graphical representations, which brings up a second question.
Has any thought been given to such aesthetic concerns as illustration and calligraphy? This was pretty common practice especially in religious contexts. The monks of Western Christendom was well known for illuminated manuscripts for a good period, and the calligraphy and beauty of copies of books (most especially the Koran) have been an esteemed artform in Arabic cultures from the Islamic golden age. Eastern calligraphy focuses on short phrases, to my understanding, but is also worth incorporating in some respects probably. While that might not be a priority for the tavern release, it does tie in to the science, art, and religion themes that his release has picked up, and tavern signage (especially in German and Japanese usage) would probably use the same code foundation.
Will there be harder books that require greater reading skill than others?
I haven't sorted that out -- it isn't just the reading skill, but the scholarly/art skills/knowledge as well. It seems like some books should require not only prerequisite skills but also prerequisite knowledge to understand at all, whereas others might actually implicitly provide prerequisite knowledge as well as the topic knowledge while imparting some skill as well, depending on the subject, length of study, etc.
It's vaguely related to the overall adoption of an innovation -- at some point, losing books/scholars shouldn't matter if everybody in the civ picks up a concept from an early age or there are representative artifacts everywhere, but that only applies to some things that lend themselves to cultural absorption/reproduction (some of the more esoteric innovations never get that way, even after thousands of years, or they occupying a middle ground where they'd be more obvious but not without work).
I interpreted this question differently, not referring to content but to difficulty of prose in a more fundamental sense. In real life, I wouldn't expect an adult or teenager with a low level of literacy to understand most translations of Kant, just because the material is difficult even if it might be a clever person who wouldn't have trouble with the concepts. There's also an issue of dialectal divergence, including things like the standard Freshman example where people mistake "Wherefore art thou Romeo?" to mean "Where are you Romeo?". But that's more complicated than what I would assume could be boiled down to a variable denoting level of prose and an accompanying level of reading skill needed for comprehension, with potentially an incomplete understanding (enough to use but not to write on) accompanied by a stressor for border cases.
Related to the dev log but not to development, there's been a lot of unsightly double-minus usage lately (--). You can easily make an n-dash (–) with Alt+0150 or, if you prefer, an m-dash (—) with Alt+0151. I like the n-dash better but either would be preferable to the double minus.