The Amazing Maurice and his Educated Rodents Discworld Hardback Library

What would happen if rats and a certain cat continually eat magically-infused orts from the Unseen University? Simple: they learn to talk. And if the cat refuses to eat anything that can speak, which seems to be a rather nasty habit among rats these days, it's time for Maurice and the rats to work together. Along with a slow-witted but kind-hearted rat piper they devise an elaborate plan to trick humans into paying the piper to get rid of one alleged rat infestation after the other, until they encounter three huge problems at once: another phoney rat piper, crooked rat hunters, and an actual rat infestation.