Our use of Speed's Oil Painting Techniques book was in part out of interest in what he had to say about a variety of painters' work.** Indeed, Dover's own press describes the book as including "expert analysis of work of Velasquez, Reynolds, Gainsborough, Hals, Rembrandt" and others. So the cover might have shown any of such works as Speed regarded to be exemplary, since he was not apparently purporting to write a book only about his own work. Nonetheless, the book curiously makes scant mention of Sargent.
Perhaps there is some Dover lore extant about the development of the cover. I've put in an enquiry to Dover's editorial department and will dutifully return with any reply I receive.
** (For the possible interest of the aficionado and collector, we similarly consulted the hard-to-find "
The Classic Point of View" by Kenyon Cox, published around the same time as Speed's book. These are essays often spare in praise and edgy in condemnation. (One well-known historical work is referred to as "immortally absurd".)