Published by the University of Oklahoma Press, and still available from them and other sources like amazon.com in a new paperback, these books are an example of wonderful format, artwork and ease of reading - however, please note that Mr. Steffan was a very skilled Western artist, horseman, collector, etc., but an amateur historian. In regards to the McClellan saddle lineage, he leaves out four distinct flavors, the M1857, M1893, M1896 and M1902. To be entirely fair, most people would have missed the M1857, but there are clear references to the M1893 in Rock Island Arsenal correspondence and year-end reports. There just no explaining the lack of info on the M1896/1902 series.
In general, Mr. Steffan seems to have relied a great deal upon artifacts he collected for models, accounts from other collectors, and specimens he examined in various museum and private collections. The books are entirely illustrated with his particular style of line art - which is quite good for general identification purposes, although you do tend to lose some of the nuances that you can only get through photographic images.
In any case, "The Horse Soldier" series is an excellent starting place, and should be considered one of the top resources currently available in material culture studies of the U.S. mounted services.