In 1818, Jacques Auguste Simon Collin de Plancy published his Dictionnaire Infernal, but it wasn't until Henry Plon's sixth printing in 1863 that the book got its now-infamous illustrations, which are a world of wonderful.
De Plancy was an atheist when he published the first edition, but became a Catholic over the intervening years and commissioned Luis Breton to create 550 illustrations of demons for the Plon printing.
I've read several articles on the Dictionary this morning and I still can't figure out why de Plancy had it out for a guy named "Leonard" — judging from the pic, I'm assuming Lenny was a well-dressed scamp who broke de Plancy's heart, either directly or by seducing de Plancy's sweetheart.
LivreDictionnaire infernal [BNF]
1863 Dictionnaire Infernal In PDF – All Illustrations Included [Occult Center]
[The Everyday Goth]