That's a long subject line.
But essentially he was the premier fantasy illustrator of his day, that being the late Victorian era to the mid 1920s. His work for Tales of King Arthur is probably the best remembered, but he also illustrated the many "Fairy" books by Andrew Lang and a ton of other stuff
His work was a pretty big inspiration for the look of knights in armor and the look of early D&D art.
Anyway, apparently people on Google+ (which I could never figure out how to use) decided to make a monster book based on his illustrations and make it available for free