For the "Monsters" atlas, I used royalty-free and public domain works of classical art and iconography. The art came from Albrecht Durer, Gustav Dore, and William Blake and the iconography mostly came from the Russian and Eastern Orthodox churches. I purchased frames from istockphoto.com and added a diffuse glow and slight distortion to each image (in greyscale). Because this was a short atlas (written all on my ipad in a couple of different hotel rooms), Caleb (my editor) and I decided we'd beef up the content with lots of artwork and casewrap it so it sort-of looked like a dr.suess book for adults.
here are some samples (there are over 100 illustrations like these in "Monsters," none of which were used more than 1 time a piece):