The Dictionnaire Infernal was written by Jacques Auguste Simon Collin de Plancy in 1818. It's seen numerous editions and takes much from The Lesser Key of Solomon, an annonymous 17th-century grimoire, which references the earlier Pseudomonarchia Daemonum by Johann Weyer.
Supposedly King Solomon wrote the first version after he successfully enslaved seventy-two demons using a magic ring (called the Ring of Aandaleeb and which bears the Seal of Solomon) and forced them to build the magnificent Moriah temple. However you can get the job done, dude.
The Aandaleeb Ring bestows numerous powers, including understanding animals, and was once owned by Adam. Before him, it belonged to the original bad boy: Lucifer.
After completing the temple, the demons were imprisoned within the finished temple. There they stayed until being unceremoniously released when unappreciative Chaldeans sacked it. Such douchebags.
The first section of The Lesser Key of Solomon, the Ars Goetia, contains descriptions of the demons. Details are slightly different in the other books.
Some versions include nifty demon sigils, and best of all, the 1863 Dictionnaire Infernal has illustrations by Louis Breton. They're wonderfully bizarre, full of character and comical menace. Quite inspirational. You'll see some of his designs pop up, albeit in somewhat modified form, later on.
Angels are invariably rendered with perfect bilateral symmetry, uniformity, and physical beauty. Good genes, basically. Demons, on the other hand, are far more varied in appearance. They represent bad genes and all the freakishness that results. After all, we naturally impute 'evil' with deformity and 'goodness' with beauty. It's a trope seen in everything from fairy tales to Star Wars. There are exceptions, such as the wicked Queen in Snow White. Even so, she becomes an old crone before perishing. There's a message in that.
Shun the bad genes, elevate the good.
Every beautific angel looks like every other beautific angel, but no two demons are alike. With both Breton and Bruegel, fish parts mesh with bird beaks, man blurs into household appliance, animal heads top lizards. Great flights of imagination. The beauty of imperfection. And that's what makes them interesting. Rather like Japanese tea cups.