Johnny and the Bomb is a children's book by Terry Pratchett. It's the third (and final) book in the series about Johnny Maxwell. A slight misunderstanding on my part had me reading the third book first. Oh well.
Johnny's world is much the same as that in which Pratchett wrote: late 20th century Britain. Except ... when Johnny is around, things often get weird. This time out, Johnny and his friends encounter Miss Tachyon, who we (the readers) know was around the same town in 1941, looking much the same. Miss Tachyon is a crazy bag lady with a shopping cart. Contact with the shopping cart sends Johnny and his friends skidding through time. A lot of the book is spent on one particular evening in 1941 that Johnny knows a lot about because it was the only time in World War 2 that a bomb was dropped on their small hometown of Blackbury, and he was assigned to do a project on it.
Goofy and mildly fun but no great work of art, this should probably only be read by hardcore fans of Pratchett who've already read the first 20 Discworld novels.