We seek books, books that might be found in a library, on the shelf somewhere, or even lost in that dungeon. Required is the title and author, a thorough report on the contents, the perceived and true usefulness, a bit of background won't hurt either. Please try to avoid the obvious "fillers" (30 volumes of Elvish heroic poetry, etc). Remember that in many settings books are very expensive; they better be worth it!
You may also include books that are mentioned in other books, works the characters never may lay hands on. Their (reported) usefulness can be amazing, and any sage worth his salt craves them desperately. Yes, those Books. A very nice and complete example is M.J.Young's Cults of the Past (http://www.mjyoung.net/dungeon/cults001.html).
On the other hand, please avoid too magical or intelligent books, as they have a strong tendency to get cliche. (Not to speak of those oh-so-overdone demonic books possessing the reader...) We seek interesting, but still ordinary books.