All clear - Connie Willis

Traveling back in time, from Oxford circa 2060 into the thick of World
War II, was a routine excursion for three British historians eager to
study firsthand the heroism and horrors of the Dunkirk evacuation and
the London Blitz. But getting marooned in war-torn 1940 England has
turned Michael Davies, Merope Ward, and Polly Churchill from temporal
tourists into besieged citizens struggling to survive Hitler's
devastating onslaught. And now there's more to worry about than just
getting back home: The impossibility of altering past events has always
been a core belief of time-travel theory - but it may be tragically wrong.
When discrepancies in the historical record begin cropping up, it
suggests that one or all of the future visitors have somehow changed the
past - and, ultimately, the outcome of the war.

Award-winning author Connie Willis returns with a stunning, enormously entertaining novel of time travel, war, and the deeds - great and small - of ordinary people who shape history.