Johnny and the Bomb
by Terry Pratchett
In this third book of the series, teenaged Johnny Maxwell and his yahoo friends uses Mrs.
Tachyon’s shopping trolley to travel through time to World War II.
. . . if you go mad, do you know you’ve gone mad? If you
don’t, how do you know you’re not mad?
Johnny and the Bomb by Terry Pratchett (Doubleday,
1996).