Courtly Love: How to Woo the King's Wife and Keep Your Head by Geoffrey Saucer
A collection of poetry from various races: human, elven, dwarven, halfling, gnomish, and some surprising ones- gnoll, dragonic, and giant poems too
Heraldry: A guide to Nobility- handy for a circumstantial bonus in researching the nobility
Works of philosophers of the campaign world, astrology, astronomy, discussions of the planes and their interactions
A collection of letters from the wizard to his or her mentor, colleagues, respected clerics and druids, and other brilliant thinkers
A collection of attempted spells, things that did not work or did not work as intended, these could be captured and modified to allow new spells
Maps, always fun and can lead to some nice plot hooks, or just help the party get to a new place for an adventure
False books- with fun things hidden inside like an imp bound to a small disk, or a statue that can be animated and enlarged with command words
The Annotated History of Water Travel in the Dulrath Empire after the Reign of Celuptus the Tyrant and Before the Coming of Grag-dur the Orc Warlord by E. Marcus Philge