From an interview with Connie Willis: "What are Blackout and All Clear about? They’re about Dunkirk and ration books and D-Day and V-1 rockets, about tube shelters and Bletchley Park and gas masks and stirrup pumps … see full wiki
I got my copy of All Clear by Connie Willis through the Amazon Vine review program after a friend recommended her writing. Fortunately, I discovered quickly that this was really the last 600 pages of an 1100 page book, and I decided to hold off reading it until after I got Blackout from the library. It made all the difference to be able to just keep reading once I had finished Blackout, as it ended at the end of a chapter, and All Clear picks up just as if there was no break (because there really isn't).
All Clear continues the story line of the main characters as they try and figure out why the time portal drops aren't working in the midst of the bombings of England during World War II. Their boss finally makes it through, but he concludes that he's also now trapped in the past. The working theory is that the flow of history was altered by their trips back in time, and now they are prevented from returning so that they can be eliminated in order to reset the proper flow of events. But there are other ways to look at how things happened, and it may be that they were kept there in order to make sure things *do* work the way they were supposed to. By the end, all the separate time and character plot lines are coming together, and everyone's having to deal with the questions about what choices they have based on what has or has not happened, both in the past and future.
I found All Clear a much better read than Blackout, as the reasons for much of what was happening were starting to be revealed. Actually, it's probably more accurate to say I liked the last half of Blackout/All Clear better than the first half, as they are really a single book. As you get closer to the end, the typical time travel conundrums start coming fast and furious, and a number of the scenes from earlier in the book play out again, only from the perspective of someone else in the group as their events intersected. If you like to dive into "but if that happened, then why did this occur, and wouldn't have that led to..." questions, you'll really enjoy the last 100 pages. :)
All Clear was a good ending to a story that probably could have been told in far less than 1100 pages. But having gotten a better feel for Connie Willis (at least better than I had with Blackout), I will definitely head back and check out her earlier work.