The Soviet High Command: A Military-Political History, 1918-1941

While not exactly a page-turner, this book is an incredibly detailed and well-written history of what it says--the Red Army between 1918 and 1941.  Political issues, evolution of doctrine, pace of re-armament, cooperation with German & other western militaries, etc. are all treated in a greater or lesser degree of detail.  I can't imagine another book of this scope will soon emerge on this topic, and in any event since the 3rd Edition (published in 2001), researchers have had less and less access to Soviet archives, so probably little scope for filling in gaps or correcting errors.