The Dungeonomicon contains essays on dungeons themselves and the creatures that populate them, as well as the social, economic, and linguistic aspects of the game. There is a strong focus on how living underground even works in the Dungeons and Dragons multiverse, complete with writeups on some of the most common races that spend their time hiding from the sun beneath the ground.
Also included are new and updated base and prestige classes, including a version of the Monk that fails to live up to its balance point (in a good way). There are even updated rules for a number of problematic magic spells and effects, notably the entire polymorph line, and a revised way to deal with the hide skill. It's all rounded out with a set of adventuring locations that help tie all of the articles and material together.
Read more at: Dungeonomicon