(In order of personal priority)
1: Increase the info on the library screen. ATM you can only see what books you have with the artefact screen, but copied books are hidden. We need a list of all the books in the libary, including the copies.
2: Expand the book list to a list that does look like it belongs to a real library. The list should include author, title, general topic.
There should be statistics, such as "number of times read" and "days belonging to fort." plus a list of readers (The old game SimCity2000 had random statistics on buildings, and there is nothing more enjoyable than that. I would love to know what the most popular book in my library is!) As well, I would want a list of readers, so I can see who read the most books. It would personalize the dwarves say if I could know that my fisherdwarf had read all the books about pulleys.
Books should be sorted by general topic (medicine, history, philosophy etc.). So you could really nerd around and go "whoa I only have 2 books on climate, I really have to raid some place with a lot of climate books!".
3: Add a noble, the Master Librarian.
The library lacks this guy in charge of it. The function should be modelled on Malachia in "The Name of the Rose" by Umberto Eco.
He'd decide which books got copied, as in he would order the scribes to copy this or that book, so his preferences would matter. (Pherhaps the forementioned book list would be acessible on his nobles screen.)
He'd issue demands like "new zinc bookshelf", for all the relevant library objects (3 chairs, chests, bags, writing materials, books, shelves, scrolls, codexes, quires). He'd sometimes forbid the export of parchment, quires, scrolls. He'd conduct meetings with scholars, scribes, other nobles, visiting scholars, resulting in happy or unhappy thoughts, depending on his personality.
He'd be in charge of the museum as well, and issue mandates for that as well. ("Bones should be displayed", "an object of brass should be displayed". etc.)
He'd double as doing a bit of work as a scribe and scholar. He should keep a few books closed to the public, because the information in them is just way too explosive for the general dwarf! (Urist has hidden "Can the Fortress Save the World" from the general public, as he deems it dangerous to read for unstudied people.")
He should have a tendency to get involved in the dark villanous plots that I do not know as I don't play that version yet, but is anything better than a Master Librarian found murdered in the library with a bloody page torn out of a mysterious book? I think not!