"The Dictionnaire Infernal (English: Infernal Dictionary) is a book on demonology, organized in hellish hierarchies. It was written by Jacques Auguste Simon Collin de Plancy and first published in 1818. There were several editions of the book, but perhaps the most famous is the edition of 1863, in which sixty-nine illustrations were added to the book. These illustrations are drawings which try to depict the descriptions of the appearance of several demons."
I first came across many of these awesome illustrations in the Time Life coffee table book Wizards and Witches, part of their Enchanted World series released in the 1980s. This was the heyday of paid tv advertising, and Time Life was famous for "get the first book in our new twenty part series free"-type offers. As a kid, I would always stay up late watching tv. Johnny Carson, original Star Trek reruns, Night Flight, and cheesy-sleazy horror and kung-fu movies were all favorites. During the commercial breaks, I'd be busy ordering whatever free crap I could. Including, at one time, a copy of the Book of Mormon. Hell, free is free!
Today, I found out that the Dictionnaire Infernal illustrations and English text are available online in PDF form here (very small download, about 2.8 MB).
If you've never experienced these, now is the time. These images and words will always be what comes immediately and vividly to my mind when the subject is demons or devils and if they can't inspire some epic old-school fantasy gaming encounters, I don't know what can.