I should know him, really, I've seen his illustrations in countless books, just never thought to look him up.
Anyways, metaphysical artist, one of the enlightened, famous for his occult and related illustrations. Reminding me a bit of William Blake, only - well, a little more refined, less spontaneous.
Link: Etidorhpa
Google Image Search: Images J. Augustus Knapp
Oddly enough not a lot written about him. The illustrator of Manly P. Hall's "The Secret Teaching of All Ages" deserves a little better than this...