A strange book which varies between interesting and (probably unintentionally) hilarious.
On the interesting side, you have stats and information on the Demon Princes of the Abyss and the Archdevils of the Nine Hells, and their respective plots and deranged mortal cultists. Plus, there are plenty of new spells, items, and prestige classes (although some of them fit into the Stupid Evil category). There are definitely some good ideas floating about, among the daftness. Also, some of the art is pretty good, if creepy.
On the hilarious side, there is the "serious" attempt at looking at the concept of Evil. Except, this is a mix of actual evil (but handled in a very simplistic manner), taboo but not necessarily evil (S&M, drug addiction....) and the most insanely Stupid Evil stereotype of some bad guy who has never read the Evil Overlord List. Like... some filth-streaked, devil worshipping "psychopath" (although the book also has its own special definition of psychopathy) who practices black magic, eats kittens, and twirls his evil moustache while tying women to train tracks and cackling maniacally.
I'm honestly not sure if I'd recommend this or not.