I got lent both these two books after a conversation on the subject.
Russia's War by Richard Overy is a very good overview of the whole of the Soviet war effort with a focus (inevitably) on Stalin and his decisions. It's a very good, moderately scholarly work.
Enemy At The Gates by William Craig is an older book (and thus misses some information that was still secret at the time of writing). It's about the battle of Stalingrad and is based mainly on information from survivors. It's a greet example of this sort of history. I was initially somewhat sceptical of the style, as Craig jumps between little fragments from each participant fairly quickly, but as the book goes on it all comes together as a coherent history. A side note for those who know the film this book inspired: the sniper Zaitsev gets all of three pages or so, but it seems that the main events in the film all basically happened...
Both books are pretty horrifying, as the atrocities committed by both the Russians and Germans in this war were totally inhuman. It's hard to even imagine the kind of hatred that could inspire such things (although I guess once it has started then it is more likely to continue).
Russia's War (Amazon)
Enemy At The Gates (Amazon)