Johnny and the Bomb
Terry Pratchett
 After Johnny Maxwell, a boy in his early teens, finds Mrs. Tachyon, an old bag lady, by a cinema he discovers that her trolley is in fact a time machine. He goes back to his town, Blackbury, during the time of The Blitz with his friends Walter, aka Wobbler, Bigmac, Kirsty and Yo-less (possibly because Johnny has been obsessing about the destruction of Paradise Street in a German raid). Wobbler gets left behind in 1941, and when they return for him, Johnny tries to prevent the deaths caused in the raid.