Red Army is the best I've read by far.
I'm actually an aspiring writer, and I can tell you now that Red Army is simply beautifully written - its a classic example of an author that knows their material well and can write with enough skill to do it justice. Fantastic piece of work
The opposite is First Clash - dreadfully written 'docu-fiction' made for the Canadian Army. Its essentially an army manual written in ficitonal form to make it easier to read. Very good examples given of real warfare, complete with a kind of summary at the end for points of interest raised by the story for officers. Worth reading if you need to know your tactics, but very poor as a story.