All Clear is the second (of two) volumes of Connie Willis' Blackout/All Clear "novel". Do not read this book until you have first read Blackout. These two volumes are not independent stories. All Clear picks up right where Blackout left off, and there's truly no help for you if you start with All Clear. Willis herself has said that these two volumes should be treated at one novel, and you do yourself a great disservice if you don't treat them that way yourself.
I really like Willis' time travel stories, her "Doomsday Book" being one of my all-time favorites. Blackout/All Clear is set primarily during the London Blitz of World War II (with some diversions to other WWII locales and times). The storyline essentially is that of historians sent back in time to study elements of the war, and who wind up trapped in their past and spend the bulk of the books trying to get back to their base in 2060 Oxford.
The storyline here has a tendency to get frustrating, since the characters have a very difficult time accomplishing the things they're trying to do. The ultimate resolution of the plot has more to do with what is done accidentally, rather than deliberately, and that tends to weaken the characters. It also means a close reading is helpful, and I'm sure a re-read (should I ever empty my to-be-read pile) will yield a significant amount of relevant detail I missed the first time around.
Nevertheless, the ending is strong and well done --- once the "frustrating" bits are over, and the characters start to get a glimpse of what's really going on, it became really hard to put the book down.
The world Willis paints of 1940s England is fantastic, and might really be the true star of the show. I don't really know if any of it is accurate, but I really got the feeling that she'd done an amazing amount of research and loved making sure the details were right.
This pair of books was well worth reading, even with the frustrations there. 4.5 out of 5 stars.