The Silver Door — Emily Rodda

Rye made it through the golden Door and rescued his oldest brother, but his troubles are far from over. The walled city of Weld in which he lives is still under attack by flying beasts called skimmers and Rye must discover who is sending them before it's too late. Also, his second brother, Sholto, is still missing and Rye can feel that he's in terrible danger. Rye must choose one of the three Doors that lead out of Weld and venture into the unknown one more time. Will he save Sholto and Weld . . . or will the silver Door be the end of Rye?