All Clear (Oxford Time Travel #4 / All Clear #2) by Connie Willis

In Blackout, award-winning author Connie Willis returned to the time-traveling future of 2060—the setting for several of her most celebrated works—and sent three Oxford historians to World War II England: Michael Davies, intent on observing heroism during the Miracle of Dunkirk; Merope Ward, studying children evacuated from London; and Polly Churchill, posing as a shopgirl in the middle of the Blitz.

But when the three become unexpectedly trapped in 1940, they struggle not only to find their way home but to survive as Hitler’s bombers attempt to pummel London into submission.

Now the situation has grown even more dire. Small discrepancies in the historical record seem to indicate that one or all of them have somehow affected the past, changing the outcome of the war.