Well-written sourcebook on adversaries for Mage: The Ascension. Book of Madness drives home the point that a human being who has sold him/herself out to the powers of darkness is by far scarier than the demons themselves. These evildoers are called Nephandi, and includes both mages who make pacts with demons for power and glory, and ones who are philosophically committed to undoing creation itself.
Also covered are Marauders, insane mages who warp reality around them into their own private universe and are immune to the cosmic backlash suffered by Mages known as paradox, an extensive essay on the nature of Paradox and belief in Mage: the Ascension, and write-ups on a variety of NPCs, mythological creatures, and demonic entities.
Book of Madness doesn't give extensive coverage on how Nephandic corruption affects the different spheres of magic in the game (other than the sphere of entropy); this is handled better in the revised edition of the book.