David M. Glantz is one of the most respected experts on the German-Soviet front, and I can't say that there is a more detailed work on the battle than his Stalingrad Trilogy: "To the Gates of Stalingrad, Armageddon in Stalingrad, Endgame at Stalingrad (which actually got split into two books itself, one for November, and the other for December through February, but it still is called a trilogy). There are some other works I could point to - Jochen Hellbeck and Geoffrey Roberts both have decent books on the battle - but Glantz is really where you ought to be looking.