Baker, A. Deborah. Up-and-Under series #4

"Since stumbling from their world into the Up and Under, Avery and Zib have walked the improbable road across forests, seas and skies, finding friends in the unlikeliest of places and enemies great in number, as they make their way toward the Impossible City in the hope of finding their way home. But the final part of their journey is filled with danger and demise. Not everyone will make it through unscathed. Not everyone will make it through alive. The final part of the enchanting Up-and-Under quartet reminds us of the value of friendship and the price one sometimes pays for straying from the path. No-one's safety can be guaranteed under the smokestrewn sky"--