"Blackout" by Connie Willis is a very good novel about time travel to WWII. The time travellers in question are historians from the future. They believe they've worked out the bugs and have now paradox-less time travel so they can go back and merely observe. One student is in the middle of the Blitz, but feels fairly safe because she knows exactly where every bomb is going to fall and when. Another, is having a thankless job taking care of evacuee children up in Northern England. Yet, another is trying to get as close as possible to Dunkirk to observe heroism in action. Soon these students and others become aware that their "perfect" time travel is becoming unwound and they are left on their own with bombs falling and the past/future changing.
This is a wonderful novel. It's smartly written and full of incredible detail. WWII, is now entering into that area of history where we THINK we know everything about the events, but in reality we've forgotten much. It's nice to be reminded of what our darkest days were like and how we made it through. Definitely worth a read!