The Paths of the Perambulator

Foster's Spellsinger series follows the adventures of Jonathan Thomas Meriweather, a law student and budding rock musician who's been transported to a world of talking animals. Here, his songs have a magical power, which he uses to aid his friend, the aging wizard Clothahump. Their latest challenge begins as an SF version of Kafka's ""Metamorphosis'': they awake transformed into crabs. Other such temporary changes offer them visions of their hearts' desires, switch their sexes, etc. It's all the work of a trapped perambulator, a powerful, primal creature that drifts through alternate universes, leaving changes behind it. Their struggle to free the perambulator and save their world is full of the usual Foster inventiveness, but the limited characters are wearing thin. This is one of the weaker entries in an amiable series. November 8