Johnny and the Bomb by Terry Pratchett
The last of the Johnny Maxwell trilogy! Johnny and the Bomb takes Johnny from helping videogame aliens (Only You Can Save Mankind), helping out ghosts (Johnny and the Dead) to straight up time travel. It all starts because Johnny was determined to help the local homeless woman after finding her in a bad case in an alley (first chapter spoiler: She had set off an unexploded bomb back in the '40s).
It was definitely the best one of the series, which makes the fact it ends there kind of worse. The characters were getting all nice and developed and Johnny was finally developing a personality (but one scene especially proves it would have been better with him being a girl instead of a white guy looking on.)
There's an emotionally painful scene in the book, involving Yo-less (who is black) and the racism back then, except it's not the people who did it that hurt him so much as one of his friend's reaction to it and it was rather on the nose.
Anyway, it was fun YA, but I still haven't recovered enough to read The Miraculous Journey of Edward Tulane yet.
I read three Diana Wynne Jones stories last night when the streets flooded and I was trapped at my dad's, but I'll talk about that later when I've composed my thoughts vis a vis 'how much is Earwig and the Witch to be blamed on the fact it's a posthumous book?'