Johnny and the Bomb
Okay, this is not Pratchett at his best. This is the third in the Johnny Maxwell Trilogy and deals with Johnny and his band of misfit friends time-traveling to 1941 England. While there they face the classic time-travel question: what happens if they interfere with history.
My biggest issues with this book is that it fails as a stand alone, feels too juvenile for TU consideration, and the humor is weak and gimmicky.