OH!!! OH!!!!! MISTAH KOTT-AIR!!!!!!!!
I'll have to read that section, Eric, but I'm fascinated by that very topic. I haven't read John Hammond's autobiography ON RECORD (and Chris & others may have some valid criticisms to make of it), but certainly Hammond was heavily involved in that of which you speak. David Stowe's SWING CHANGES has some valuable insights to offer (and Stowe has written a wonderful essay about Cafe Society as a gathering point for artist-leftists in the late 30's/early 40's; unfortunately it's not on the web, but I have a copy of it and could mail it to you), and Michael Denning's THE CULTURAL FRONT talks extensively of the connections between jazz and left politics in the 1930s and early 1940s.
I don't have the Mosaic Commodore sets, but don't they contain lengthy interviews with Milt Gabler? Perhaps there he discusses the left/jazz alignment of that time... also, the From Spirituals to Swing 3-CD Vanguard box is a real treasure trove, if you don't already have it... It reproduces the original 1938 program, replete with ads for socialist bookstores, Spanish Civil War benefits, etc.
I realize I may be ranging farther afield here than the subject of your original inquiry... in any case, I'll try to post more books/essays as they come to me, but those are some of the sources that first come to mind.