Blackout and All Clear by Connie Willis (2010)
These two volumes follow the adventures of Michael, Polly, and Eileen, thee time traveling historians who have gone back to 1940 to observe how the average Londoner withstood the Blitz. Armed with knowledge of the exact time and place of bombings, the three should be safe observers.
In a time of crisis, though, how can anyone remain an interested but dispassionate observer? The feel for WWII era England is wonderful and, though very long and detailed, this is the sort of book you can lose yourself in.
For even more novels set in World War II check out our bibliography.