"Horsemen" was created by Escher in 1957 when he was commissioned by a Dutch bibliophile book society, 'The De Roos Foundation' to write and illustrate a book about his symmetry artwork. The book's title in Dutch was 'Regelmatige Vlakverdelig,' or 'The Regular Division of the Plane.' The book has been reproduced and translated in several of the important books about Escher's work, and this is considered the most important document containing the artist's explanation of his methods and inspirations. 

As I understand it,  The deRoos Foundation published a fine book now and then on a subscription basis for its members only. The edition of the book was only 175 examples, though it is hard to imagine today a fine, handprinted book created in an edition of only 175. But so it was, and each book was numbered from the edition and carefully printed with full-page woodcuts by Escher created directly from his hand-carved woodblocks. This was one of the few times when Escher explained himself at length in words and pictures, so this is a critical document which came at the peak of Escher's creative powers (he was about sixty years old at the time, and had been working with symmetry designs for over twenty years). Escher has written that he was flattered by this commission, that the book was beautifully produced, and that he felt he had been well-compensated for his work.