The Dicionnaire Infernal by Collin de Plancy is a must-have for those of us intent on keeping our demonology knowledge ship shape.

The full text is available for PDF download. One may also view all of the illustrations by Louis Breton (engraved by M. Jarrault) separately at Wikipedia Commons.

A fantastic key (in English no less!) with notes from additional  sources is available at Deliriumsrealm.