- Each Geronimo Stilton book is fast-paced, with lively full-color art and a unique format kids 7-10 will love. New Mouse City was in danger! An evil professor was threatening us all with his terrible inventions. The authorities called upon me, Geronimo Stilton, to thwart his plans by traveling on a top secret mission - to outer space! I was totally unprepared to go. But moldy mozzarella, my trip was out of this world!