Book of Vile Darkness

Great sourcebook for when you need extra mature options for the villains your players are going to face. While the appendix does give you options for playing such villains, I think the options were designed with NPCs in mind.

It packs in lots of options, some of the interesting ones include two new subraces, lots of evil spells, your typical assortment of feats, and new magical items; all themed with evil and horror. There's also background, stat blocks, and plot hooks for the demon lords of Abyss and the diabolic rulers of the Nine Hells.

Some of the poorer bits include the small section on new monsters. I think it's minor and uninteresting enough that they shouldn't have bothered, and used that space for fleshing out the other sections.

The prestige classes were pretty poor I think. The majority of them were meant to be themed to their respective demon lord or devil prince, but I didn't find them coherent or thematically interesting.

What I would have liked to see more was more evil ideas and hooks. For example, the options on drug use and torture was useful, but would have been better if ideas on how to use them were also present.

Overall though it's a great resource for options you could throw in sparingly into your campaigns, even if you don't want to deal with the subjective morality questions raised in the book.