Blackout is a work of historical science fiction by Connie Willis, a continuation of her earlier time-travelling-historian books that started with Doomsday Book and To Say Nothing of the Dog. It can be read and enjoyed even if you have not read these earlier books.
Blackout takes place (mostly) in England during WWII; it starts out however, in 2060, where time-travel technology allows historians to go back and study history as it happens. At the moment, quite a few historians are setting out for the years of 1940-1944, to study the events around WWII. Three of these, Michael Davies, Poly Churchill, and Merope Ward, are studying events surrounding the Blitz -- Michael is studying the Battle of Dunkirk, Polly the reaction of Londoners to the Blitz, and Merope the lives of children evacuated out of London. And that's what the story is about.
Okay, there's a bit more to it than that -- a lot more -- but that's actually a lot of the story; what it was like in England during the war. This is by no means boring, and the background is exhaustively researched and the story well written. Of course, this is the story of historians from the future, and they have all kinds of difficulties with temporal schedules and learning how to live like 1940s contemporaries without acting so odd that they get reported as possible German spies. This being a Connie Willis story, any number of things go wrong, and this being a war, some of these things are fairly dangerous.
Eventually it becomes clear to the protagonists that some of the things going wrong can't be explained simply by the bloody-mindedness of time-travel technology and the chaos of a country under siege. All of the theories say that it is completely impossible for a time traveller to change history, but as the chaos mounts and they lose contact with the future they start to suspect that the theories might be wrong.
This is a very good book, and well-worth reading despite the material being rather familiar to Willis fans. WWII doesn't get boring, and if the Blitz is well-trodden territory, that's only because it deserves to be. That said, the setting, style, characters, and tone of this book are just what we've come to expect from Willis; this book fits right into the mood of her earlier books (although it's not as dark as Doomsday book)
Unfortunately, this is actually a 1000 page novel split into two parts; I strongly suggest that you buy both Blackout and the sequel All Clear together, as Blackout doesn't come to any sort of ending point... It just stops. And it's plenty good enough that you won't want to stop.
I would recommend this book to anyone who likes humorous and well-researched historical fiction, although I recommend buying the second book, All Clear at the same time; you will not want to take a break between the two. If you are uncertain whether this book is for you, I would recommend starting with the less-massive To Say Nothing of the Dog, or even (hey, why not) the first book in the 'series', Doomsday Book.