Book blurb says: Magnus was born big and he won't stop growing. His first and favorite word is "More!" - a request that runs his frantic little parents ragged searching for bigger and bigger portions to feed their giant baby mouse. When Magnus is carried off by Jim the Rat Catcher, his parents are heartbroken; but magnus's advetures have just begun.
I say: One of my favorite authors. Dick King-Smith fits into that unnamed genre which is written for all ages but winds up being listed as a children's book. Interesting, fun story.