The Amazing Maurice and His Educated Rodents A Discworld Novel Terry Pratchett 2001
Maurice has survived four years on the toughest streets of Discworld. He reckons that rats are dumb, but he's smart enough to realize that there's a new kind of rat around, rats who can talk.
This smart cat gets his own plague of rats and a Pied Piper, but are there any ordinary rats left?
