Be aware before you start this book it is not complete! To finish the story you also have to read "All Clear". Frankly the story could have been told in one book not two. I think it was a matter of making more $. Sad.
If you like reading about the Blitz Blackout/All Clear give a lot of bit that really give a viseral context to the era. But it the Blitz is NOT your passion plowing through these two book is a real trial. Why? Mainly because you will encounter 3 of the most annoying characters it literature: Eileen, Binnie and Alf. Elileen is a whiner, Binnie and Alf are the monster children that are like the perpetural fingernails on the blackboard. The author for gawd know's what reason makes the story finally revolve around them in the final half of the story. Painful, like poking a sore tooth. I am one of those folks that feels compelled to finish a book (or story) one I start it. I hesitate to start another of this authors books for fear these are the type of characters that are close to her heart.
Probably the most important thing to know about this book is that it is really half a book. It ends with events completely up in the air with a "to be continued" on the last page. Oxford in 2060 has historians being sent into the past to observe events on a regular basis. The main characters in this book are in various times and places (London during the blitz, the evacuation of Dunkirk, etc.) of WWII from 1939 to 1945. They have been doing time travel for 40 years at Oxford, but for the historians in this past era things begin to go wrong and they have no way of knowing why. It's an interesting premise, but until the completion of the next book "All Clear", it's difficult to give an opinion overall.