Douglas B. Green (aka Ranger Doug, aka The Idol of American Youth) of Riders in the Sky has written the definitive book on singing cowboys. I loved it. There's great information on every page about singing cowboys, singing groups that appeared in cowboy movies, and much more. In fact, there's more in this book about the Durango Kid movies that I've ever seen before. From the obscure to the famous, Green covers them all.
For example, did you know that Bob Steele sang in several cowboy movies three years before John Wayne appeared as Singin' Sandy? And did you know that Steele's twin brother dubbed Wayne's singing in that movie? (Others may tell you differently, but I trust Green.)
The Sons of the Pioneers are covered in depth, and Green devotes seven pages to a timeline with the group's different configurations.
Tex Ritter, Monte Hale, Rex Allen, Roy Rogers, Gene Autry, Dick Foran, Bob Baker, Vaughn Monroe (yes, Vaughn Monroe), Whip Wilson (you didn't know Whip was a singing cowboy?), and on and on. If you, like me, spent many of your Saturday afternoons in a crowded theater listening to singing cowboys, you can't go wrong with this book. And did I mention the great illustrations? Check it out.