The artwork on the covers and frontispieces was done by three successive artists. For views of the covers of the books, please refer to the section in this web site called "The Books."

The much-admired Walter S. Rogers contributed his quality artwork through volume 10. This prolific artist contributed about 700 paintings and drawings for over 300 series books, nearly all of them published by the Stratemeyer Syndicate, beginning in 1911. It is curious that so little is known about him personally. His work ends suddenly in 1931, so it is a safe assumption that he died or retired in that year. His covers for the X Bar X series are in the white spine format, and, like all of his work, are noted for detail and subtle use of color and shading.

J. Clemens Gretter illustrated volumes 11-15. Gretter's covers feature bolder color and less detail than Rogers'. The spines on volumes 13-15 move out of the white spine format and show a little design and color. The endpapers throughout the series, once it was bound in red, are also by Gretter (the endpapers of the gray editions are blank). They show two cowboys roping steers, with one cowboy in the foreground having roped a shorthorn in such a manner that the unfortunate animal has been thrown on the top of its head.

Paul Laune stayed with the series from volume 16 to the end. His work presents us with wrap dust jackets with imaginative use of color, including orange, purple, and even lime green. His scenes and figures are the most realistic of the three artists. Glossy frontispieces ended with volume 17. However, Laune did not use watercolor wash in his frontispieces, as his predecessors did, but rather pen and ink line drawings.