All Clear is the second half of the story Connie Willis began in Black Out, about historians in the future using time travel to research historical bits that catch their eye. World War II is a popular research destination, and the story follows three students who find themselves trapped in their research posts when their retrievals go awry. The first book is spent with them wandering about separately; in this book they come together except when they go apart again. And then there are the non-chronological bits, as you expect in a time-travel book.
Willis always has strong page-turning abilities, so despite the huge numbers of pages the story moves quickly. I'm not sure why it was two books; it seems like the story could have fit more neatly into one, as every point was made several times over. Several times things went wrong because one person knew something but didn't share with the others, and only realizes too late what a mistake that was. The first time I can accept, but they did this over and over again, without ever learning from the past. Usually the reason for keeping a secret was to avoid worrying each other, which became even more silly as they face the increasing problems of the blitz.
Another few hundred pages were spent on them angsting over having changed the future, which struck me as extremely silly because they had inserted themselves into the past on purpose, after having discussed for pages on why it was safe. Then they (and especially Polly, the main character) notice that they do actually have effects on people -- they talk to someone and make them catch a later bus, for example. Sometimes that's the difference between life and death, if a bomb lands just right. Perhaps they've affected the future so there is no one to pull them out? And so now the Germans will win WWII? But it never made sense to me, especially with all the talk about time as a chaotic system. Of course sometimes people miss a bus. Get back to details of shop life or something.
Willis is wonderful at tight connections, especially ones just missed. The night St Paul's cathedral almost burned down is done brilliantly, with all three characters weaving crazily in and out of danger and history and close encounters with other historians, but never quite getting rescued or killed. Mike's contrast between his initial thesis on "heroes in history" and his actual understanding of people in war touched me. One of the best parts was watching the historians move from treating the people they were studying from test subjects (the 'contemps' do such and such) to actually believing they were worthy of consideration and respect. Sometimes I just wanted to shake the time-travelers for their arrogance, which made their final actions even better.
Oh, I don't believe in Polly's final love story either, but it didn't seem that important. B-