A Visitor's Guide to Hell - A translation of the Chinese version of what happens to the human soul after death [with some illustrations]. [more inside]
The Dictionnaire Infernal. Tired of the same old devil costume each Halloween? Why not go as Bael, Asmodeus or Gaap? Written and (wickedly) illustrated during the 1800s, the Dictionaire (annoying sound warning) has the demon you need. Of course, there are other resources. If you have a hard time choosing, try your birth month's demon.
Infernal Device 49 Examples of Human Perversity