The Amazing Maurice and his Educated Rodents (Discworld Novel 28)

It's not a game any more . . .

Every town on Discworld knows the stories about rats and pipers, and Maurice - a streetwise tomcat - leads a band of educated ratty friends (and a stupid kid) on a nice little earner. Piper plus rats equals lots and lots of money.

Until they run across someone playing a different tune.

Now he and his rats must learn a new concept: evil . . .