If you want THE single volume book on WWII, an updated 10th anniversery edition of
A World At Arms was just released. Only for people already familiar with the war, if you really want to understand why everything happened in context, there is no better book. It's a VERY long read, very dense.
Also:
The Great Influenza
A Short History Of Nearly Everything
Krakatoa: The Day The World Exploded
Eisenhower: A Soldier's Life