It's hard to pick favorite works by a favorite author. With Connie Willis, it's even harder. Bellwether is a fine example of how she crosses science fiction with a romantic comedy of errors; Doomsday Book showcases her ability to tackle serious subjects with finesse; To Say Nothing of the Dog is just a fun, crazy romp about time travel and Victorian England. They're all good, but for some reason, her best work centers around England during World War II. It's no suprise; she's done so much research into the time, and clearly has enough knowledge and interest in the subject to write well about it.
Blackout covers that same subject, but like the differences between "Jack," "Night Watch," and "The Winds of Marble Arch," it does so without seeming like it's an imitation of those stories. For one, Willis populates the novel with three different time traveling historians for Oxford; for another, it covers the Blitz in great detail. In fact, she covers the subject in such great detail that this is only the first half of the story. The rest of it won't be revealed until this fall.
That underlies my biggest gripe with the book. I'm not a very good series reader, because I forget a lot of details from one book to the next. By the time All Clear is published, I will have forgotten most of what happened to all the characters. This is partly due to the fact that there are three historians, whose stories are told more or less simultaneously, and who typically go by two different names during the story. They have their real names and their assumed names, and it makes it hard to follow in parts. Additionally, since this is only half of the story, it means that the novel is all setup, with very little resolution. It's still a good read, and it means that you will really care about the characters by the time you get to the end of the book, but had I known this story would be told over two books, I would have waited until I had them both before reading them.
Still, this isn't to say that I was disappointed. The setup was well done, and the events were brought together in a sensible way. I'm eager to read the rest of the story to see how it resolves. I just hate that I will have to wait until the end of the year to do so.