This was a really cute story. Basically, a bunch of rats, snacking on wizardly refuse, attain sentience. They also hook up with a sentient cat. They can all speak, learn, and chat with each other, and at the Cat's devious notion, dupe a "stupid looking kid" to play the role of a rat-piper, moving from city to city where the rats act up, the Piper comes in and clears the rats out, and they all get paid.
This time, however, they've stumbled into a town where there's something really evil going on, and all the wisecracking cats, tapdancing rats, and stupid-looking kids in the world might just be in over their heads.
Well written, with a bit of whimsy in nearly every chapter, this was my introduction to the Discworld series, and I dare say I'll be back. The wonderful observations from the rats point of view are fantastic (there's a great part where one of the rats is asked something along the lines of: "Do you know what animal swarms into a place, breeds terribly, spoils everything they can't use and wastes everything they can until there's nothing left?" and the rat says, "Sure. Humans.")
The story gets a bit dark in places for a young reader, though a teen would probably get a laugh. And the mythology lover in me adores the play on the Pied Piper of Hamelin - like Orson Scott Card's "Enchantment" did for Sleeping Beauty, Pratchett did here for the Pied Piper of Hamelin.