The importance of the German loss at Stalingrad is rather overestimated in popular culture. It was certainly a big loss, and certainly a psychological turning point for both Allied and German perspectives, but strategical and operational it was not that important as it seems.

Basically the war in the east became "un-winnable" after the Battle of Moscow. Really the only hope that the German Wehrmacht of winning this war had was ending it as quickly as possible. As /u/EdMilibeard pointed out, the Soviet Union had serious advantages over Germany. They had the help from the western allies, they had the inner lines, they had more recruits, more industrial power etc. Their bigges weakness was the inexperience of their officers and lacks of modern equipment in the early stages of the war.

So when the Battle of Stalingrad began, there was little hope of winning on the eastern front for the Wehrmacht. And then came horrible, horrible, decision from Hitler and the High Command : Starting the operation in the Caucasus at the same time meant there were not enough resources for either of those operations to be succesful. But now the 6th Army at Stalingrad had to hold it's ground, so that the Heeresgruppe A doesn't get encircled.

The losses that the Germans suffered at Stalingrad were severe, but the Soviet lost much more. Everything added up the Red Army deployed 1.7 million soldiers at Stalingrad (against German ~900.000), losing more that a million (half of that KIA) (German losses some 300.000).

In my humble opinion the most important turning points in the eastern theatre were :

• Battle of Moscow

• Battle of Kursk

• Operation Bagration

The first indicating the war can't be won, the second robbing Germany of operational initiative, and the third marking the complete breakup of German forces in the east.

Here are some articles/books to read up on (Some of them are in GErman, you'd have to look if there are translations for you available, but I guess there aren't)

Michael Salewski: Kriegswenden: 1941, 1942, 1944 (Mitteilungen der gemeinsamen Kommission für die Erforschung der jüngeren Geschichte der deutsch-russischen Beziehungen)

Manfred Hettling: Täter und Opfer? Die deutschen Soldaten in Stalingrad. (Archiv für Sozialgeschichte)

Richard Overy : Russia's War: Blood upon the Snow

Germany and the Second World War : Especially volumes four, six, nine and ten

/e Another one

Richard Overy : Why the Allies Won.

Well one other significant factor is the climate, the Autumn rains in 1941 really bogged down the latter stages of operation Barborossa and delayed the German advance into Russia proper. Then came the notorious Russian winter which, considering the Wehrmacht were still in summer uniforms, devastated that German invaders and handed the Soviets a significant advantage. The weather ended up being a big help for the Soviets throughout the war as the winter snows made any hope of a German airlift to save the troops surrounded in Stalingrad nigh on impossible.

Another factor is the disorganisation of the Nazi state. Now, most people associate Germany with efficiency but the Nazis were notoriously disorganised. Most decisions made by the Nazi government weren't based on any official written orders from Hitler but instead were made on the basis that it was probably what the fuhrer would want, regardless of whether he explicitly ordered it or not. This led to some dire inefficiencies in Nazi industry and meant that the Soviet's industrial advantage over the Nazis would always remain in place.

Then we can come into the Soviet advantage of manpower. Nazi Germany had a population of about 69 million in 1939 while the Soviet Union had a population of probably about 250 million. This simply meant that the Soviets could more easily replace losses incurred on the battlefield while the Nazis simply found them to be irreplaceable.

Next on is geography. Obviously you've seen a map of the Soviet Union in 1941, it is huge. Yet, Nazi Germany, even including its conquered provinces later in the war was significantly smaller. This basically meant that while a 250 mile advance in the Russian steppe would lead to little bit overstretched supply lines and a lack of shelter a similarly large advance into Nazi Germany from the 1941 Soviet Border would have brought an invading army scarily close to Germany's heartland.

As well as that is resources, the Soviets had vast natural resources on top of vast space while the Nazis had to import many raw materials even after years of attempting to create a programme of self-sufficiency.

Edit: Book for the first part would be Antony Beevor's 'Stalingrad', Ian Kershaw's books on Hitler, 'Nemesis' and 'Hubris' cover the terrible coordination of the Nazi state and Chris Bellamy's 'Absolute War' would give more detail to the comparisons of Soviet and Nazi industry, manpower and geographical factors.