The Napoleonic War and WW2 taught the Russians that they need neutral countries between Germany and Moscow to act as buffer states to survive a land invasion till the Russian winter sets in. This explains why NATO wants Ukraine to be part of NATO and why Putin opposes it.
The best book on the Eastern Front in World War II is When Titans Clashed by David Glantz and House.
It was published in 1995, but no book since then has surpassed it in terms of clarity and conciseness.
At only 400 pages, the authors begin by explaining the capabilities of the German and Red armies before 1941, before describing the war in three time periods from June 1941 to May 1945.
This is the book to read if you have time to read only one book about the Eastern Front. I read it in 2003.