The Amazing Maurice and his Educated Rodents by Terry Pratchett
Even wizards produce leftovers.
But a wizard's rubbish is laced with magic, and for the rats that forage this rubbish, the magic has changed them - they can speak and read, and have rather grand ambitions for a comfortable retirement.
Which is perfect for a con-cat like Maurice. He has his own magical talents, and wants to get rich quick. Together with the rats, and young (rather simple) Keith, the 'piper', they work the towns to create their very own plague of rats - then lure them away for cash.
But in the run-down town Bad Blintz, this little con goes wrong, and suddenly these educated rodents aren't playing to the piper's tune . . .