W.W. Denslow
The Library of Congress mounted a major exhibit on the centenary of the publication of The Wondeful Wizard of Oz and maintains a site related to that exhibit, covering the books, L. Frank Baum and his illustrator, W.W. Denslow.
The AIGA (American Institue of Graphic Arts), has a nice essay on Denslow's work.
W.W. Denslow's Wikepedia entry.
Don't forget that prints from Denslow's works are available through the Art for Children section of our homepage.