Foster, Alan Dean. Spellsinger Series #bk. 5

Jon-Tom the spellsinger and his friends venture into the high mountains to free a cosmic creature whose captivity threatens to drive the whole world mad It's an ordinary day in the Bellwoods, save for the fact that Jon-Tom the spellsinger has awoken as a six-foot blue crab. He soon reverts to normal-as normal as a college student trapped in a world of magic and talking animals can be, anyway-but the bizarre changes keep happening, affecting not just him but the whole of this strange world. His wizardly mentor suspects these are the effects of the perambulator, a cosmic being whose presence distorts reality. One has been imprisoned in the mountains, and unless they free it, it will rip the world to shreds one insane illusion at a time. So Jon-Tom sets out, backed by the wizard, a boozehound owl, a gutter-minded otter, and a warrior koala with a taste for leather. Saving the world will be a cinch, as long as they don't lose their minds along the way

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