"No one knows his name. Not one piece of his original art appears to exist. If you discount the fact that the magazines he worked for were reprinted until the early 1920s, none of his art has ever been reprinted outside some scattered fan magazines and websites. Yet from roughly the summer of 1901 to the winter of 1910(and possibly beyond), he produced well over 3,000 covers for some of the most popular juvenile periodicals of his day, the equivalents of the modern comic book."
Larry Latham shows around ninety covers.
(link via Dr jon)