Jack Snow was an absolutely brilliant illustrator -- he did every book after the first one, for a very, very long time. After L Frank Baum wrote the original 14, a few other authors picked up the mantle (Ruth Plumbly Thompson being the most prolific by far) and Jack Snow did all of those, until he authored a few books himself.

Sadly, none of the other authors quite captured the surrealism, magic, and originality of the original 14, but many of them were still fun to read.