Diane Duane - A Wizard Alone

Kit and Nita join forces once more against the terrible Lone Power on an unusual battleground, as they fight for the heart and mind of a young wizard with the power to save their world…
Initially, Kit finds himself flying solo as Nita has sunk into a deep depression after the events of The Wizard's Dilemma. Luckily, Kit's telepathic pooch, Ponch, is happy to fill Nita's niche temporarily, as long as enough dog biscuits are involved.
Kit's fighting to understand why autistic wizard-in-training Darryl McAllister has been stuck in his wizardly Ordeal for over three months. Is it merely the fault of his autism? Exploring inside Darryl's mind, Kit and Ponch discover complex landscapes of weird beauty that belie Darryl's rocking, vacant exterior. But they also find the Lone Power, attacking Darryl with a relentless brutality that seems to make little sense. What makes Darryl so important — and why can't he escape his Ordeal?
Nita, meanwhile, is distracted from her sorrow by a series of strange dreams in which some mysterious being pleads desperately for her aid. What do the cryptic messages mean? She has to find out quickly — because now Kit, too, has vanished. Can she find him — and if she does, will the two of them be able to come together in time to deal with the Lone Power before It can destroy them?