With WWII books in particular, you have the problem with the general vs the specific. There are plenty of good general histories of the war--John Keegan's is more than adequate--but by their very nature don't have the sort of visceral, specific details that make for good gaming inspiration.
Two that come immediately to mind are A Bridge Too Far, the classic book on Operation Market-Garden; and Iron Coffins, a memoir of one the very, VERY few German submarine captains to survive the war. Enemy at the Gates: The Battle for Stalingrad is very readable, though it may suffer a bit in the fact-checking department. (If I recall correctly, it recounts the historically-unconfirmed sniper duel between Zaitsev and Konig--the basis for the movie--as fact. There may be other "grandpa's war stories" inaccuracies as well.)
