Steven Utley wrote:
Here's some of what I'm talking about.  The Four Color series never had nearly the line-up of famous authors that Gilberton's Classics Illustrated had, but Dell adapted works by Johnston McCulley, Edgar Rice Burroughs, Zane Grey -- authors whom Gilberton evidently deemed unworthy of inclusion alongside Homer, Shakespeare, Dickens, and Longfellow. 


Of course, Gilberton may have regarded McCulley, Burroughs and Grey as being the sort of authors who were in copyright and might want a royalty.