A Wild Sheep Chase by Haruki Murakami

A New York Times bestselling author—and “a mythmaker for the millennium, a wiseacre wiseman” (New York Times Book Review)—delivers a surreal and elaborate quest that takes readers from Tokyo to the remote mountains of northern Japan, where the unnamed protagonist has a surprising confrontation with his demons.
 
An advertising executive receives a postcard from a friend and casually appropriates the image for an advertisement. What he doesn’t realize is that included in the scene is a mutant sheep with a star on its back, and in using this photo he has unwittingly captured the attention of a man who offers a menacing ultimatum: find the sheep or face dire consequences.