First I should say that in general I dislike time travel stories. They always seem to have some sort of cheat in their resolution. I also am not a big fan of WWII fiction. That being said I completely enjoyed Blackout and All Clear by Connie Willis. These books tale place in the same universe as her earlier time travel novel Doomsday Book
Willlis is a brilliant writer. She is one of the most award winning SF Writers of all time. One of her strengths is writing novels and stories that both SF fans and the general public enjoy. She has done it again with these books.
Blackout and All Clear follow a handful of Time traveling Historians who are all observing England during WWII. They are looking into several different key events, The Blitz, Dunkirk, and the VI and V2 attacks. Things are going well at first, then suddenly they all find out that their drop windows won't open and they can't get home, and things seem to be happening is slightly different order than history believes. Time travel theory says this can't happen. A historian can't be trapped in time, and history can't be changed. Historians can only go back in time to a time an place they can't change. If there is a possibility the could change things the time drop will not open and let them go back in time.
As time moves forward the various historians path's cross. The plot moves between their day to day cover lives in wartime London, where they walk a fine line between being observers and being participants in what is going on around them, and Oxford in the future where their colleges are trying to find out why they have not come home.There is a slight cheat at the end of the story, but the plot and characters are so good that I didn't care.