Fortress of Gold: Book Two of the Magicians Gold Series

Sixteen-year-old Pran travels with three companions to the mythical world called Earth to replenish their gold, which powers magic in their world. When they arrive in New Jersey, a prophecy directs them to the daughter of two Princeton professors, who will be their native guide. In order to save their world, Pran and his friends must pull off a daring daytime heist of the most heavily guarded building on Earth: Fort Knox.