In tonight's adventure, my player's found Room 53 of the Sunless Citadel, Belak's study. While one book in particular is specified, I decided to work up a few others to add to the encounter.
The Teachings of Auk, is a scroll on papyrus, written by the Archdruid, Auk the Tender. It is an exploration of nature and the a list of 71 tenets he lived by. Any druid reading it may learn detect poison and Disease, darkvision, and spike growth.
The Eternal Cycle, is a small book, bound with ash wood covers with bleached leaves as pages. It is written by a druid who turned to necromancy, Deana the Pale. She explores the essential nature of life and death and discusses why druid's use necromantic spells, such as the blight spell. Any wizard or sorcerer reading it may learn the chill touch cantrip, as well as the spells false life and ray of sickness.
The Cthonic Verses, is a large book, bound with black leather and sigils in Primordial revealing it's name. It's author is unknown and the work explores the fallacies and failures of druidism, claiming that the earth is a powerful, ancient and dark master who has little tolerance with the fleas that scurry upon it, the humanoid races. In truth, only the strong should survived and the weak deserve nothing but to be preyed upon.