Sleator, William - Marco's Millions
Second-hand books in good condition. Images sourced from the internet. All books are cleaned before shipment/collection.
Marco has always been obsessed with travelling. When his younger sister, Lilly, discovers an invisible tunnel in their basement, he can't resist crawling into it. What he discovers there is truly fantastical - a world inhabited by insect-like creatures. They communicate via telepathy, and tell Marco that they need Lilly to help them contact The Unknowable (their 'God') and thus prevent their world - and Marco's - from destruction.
Lilly is too scared to venture in to the hole, so Marco agrees to go in her stead. Once there, he must take a mysterious box on a long journey to the Unknowable - a singularity that acts as a gateway between millions of different universes. When he discovers a second box, he is given the power to control gravity and time. And, at the gateway of the singularity, with its numerous adjoining universes, Marco knows that he really can go absolutely anywhere . . . .