If you read the Johnny Maxwell trilogy, you can actually see Pratchett's undeniable talent blooming. In the third book, Pratchett gives us a smart person's time travel fantasy, full of hilarious situations and interesting info on the trouser legs of time. (Don't ask)
Old bag lady Mrs. Tachyon is considered nuts but harmless, pushing around an enormous trolley full of bags and a mangy cat. One day she's found unconscious and beaten in a street; after Johnny and his pals have her taken to a hospital, they put her trolley in Johnny's garage for safekeeping. But Johnny soon finds that the trolley has more than garbage -- there's a new newspaper dated from decades ago.
Before you can say "what the disc!", Johnny and his friends (dignified Yo-less, wannabe-nerd Bigmac, abrasive Kristy, and not-so-dignified Wobbler) are whisked back in time to 1941. At first they're intrigued by the weirdness of the old place, but things take a nasty turn: Bigmac is arrested, while Wobbler is first harassed by a bratty kid and then accidently left behind. When Johnny and his pals reappear, they soon discover that their brief trip back in time has completely messed up the timeline...
This book is more complex than "Johnny and the Dead" and better-written than "Only You Can Save Mankind." Pratchett's quirky characters, occasional social commentary and funny speculation (the trouser-legs-of-time description is the best time description you can find). The appearance of the elderly Wobbler is a stroke of genius, as is the "... I'm a Muslim" joke that serves an important part of the plot.
Pratchett's writing is clearer and quirkier here than before. The storyline is far smoother and more detailed, and he puts in extra scenes that add to the characters (such as Yo-less dealing with a '40s woman's racism) without distracting us from the story. And the funnier scenes (like the police interrogaton, or the "spy!" harrassment) are absolutely hysterical.
As before, Johnny is the one really normal person as well as the smartest. Yo-less makes less of an impact unless dealing with stereotypes, and Bigmac makes very little unless being interrogated. The rather self-satisfied Kristy barges undiplomatically at Johnny's side (though she does deal with '40s sexism in a very amusing way), and Wobbler has the subtlest and perhaps most interesting role.
"Johnny and the Bomb" incorporates the strengths of the previous two books, finishing it off with a flourish (and lots of explosions). Funny, cute, a time travel story for the thinking reader.