Johnny and the Bomb
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It's been a long time since I've read this series, but they were fun to go back to. In this book, Johnny (previously able to communicate with video characters and the dead) develops the ability to move through time, and tries to prevent the deaths of a street full of people in WW2. Quite a few of Pratchett's ideas about time travel in this book later showed up in the Discworld series - especially in Thief of Time, of course, but I recognised a few echoes from other stories as well.