Hi Tina
I first came accross Terry Pratchet reading the trilogy - Truckers, Diggers & Wings to my kids - REALLY good!
Then went on to read the entire Discworld series - It is not Sci Fi or Fantasy - he just puts characters in a world where normal logic can be turned on its head. He makes you think.
The characters in Johnny and the Bomb are in two other books - "Johnny and the Dead" and "Only you can Save Mankind" - Great stories - got my kids into reading which cannot be bad!