The Dragon's Apprentice (Chronicles of the Imaginarium Geographica)

Seven years after the events of The Shadow Dragons, John, Jack and Charles are finally able to return to their beloved Archipelago of Dreams. But even as their return is celebrated by old friends, new concerns shadow the reunion: the threat of Ecthroi, primordial Shadow. And perhaps even worse, the apparent splintering of Time itself.

Now, the Caretakers must fight against their most fearsome enemy ever and attempt to restore Time. They must journey through a forgotten Door from the destroyed Keep of Time in order to seek out the Dragon's Apprentice. If they fail, it will mean the end of both of the worlds. But success will carry its own price--a price that may be too high even for the Caretakers to bear.