When we start researching about a new topic it is usual to not know from where to start researching. The Eastern Front of the Second World War (or Great Patriotic War) was an extremely long and violent combat between two Great Powers. That is why many books, articles, thesis, etc. has been written about it. These two are the ones that I recommend to start investigating:
When Titans Clashed: How the Red Army Stopped Hitler, David M. Glantz and Jonathan M. House
David Glantz is, at least for me, the best historian of the Eastern Front. He has written many books and articles about the topic and most of them are reference works. When Titans Clashed is the summary of the entire war and it is a great introduction to the four years long conflict.
In addition, as Glantz has written many books about the war (most of them extremely interesting), it could be helpful for the beginner to get used to his style soon.
Absolute War: Soviet Russia in the Second World War, Chris Bellamy
Another great work about the war. Bellamy focused on the first two years of the war, providing many details of those two years but goes through the last two years quite fast. Anyway, it is a very good book as it provides some statistics about the war that could be very useful for a beginner, as he/she could start realising how big this conflict was.
For example, Bellamy gives some tables regarding Soviet and German casualties in some of the major battles and he explains a extremely delicate topic: Soviet casualties during the whole war.
After these two and when you have some idea about the development of the war, you will be able to start reading more specific and dense books. As I said before, David Glantz is my favourite historian but in this case I do not recommend his books until you know not only the general idea of the war but some more details. His books are extraordinary but also quite dense (he specifies divisions, brigades, regiments and even battalions while talking about a battle!).
In the future I will publish some posts about the bibliography of different battles of the war so the people interested in a specific battle knows which books to read.