The Fifth Head of Cerberus by Gene Wolfe
Far from Earth two sister planets, Sainte Anne and Sainte Croix, circle each other. It is said that a race of shapeshifting aliens once lived here, only to become extinct when human colonists arrived. But one man believes they still exist, somewhere out in the wilderness.
In THE FIFTH HEAD OF CERBERUS, Gene Wolfe brilliantly interweaves three tales: a scientist's son gradual discovery of the bizarre secret of his heritage; a young man's mythic dreamquest for his darker half; the mystifying chronicle of an anthropologist's seemingly-arbitrary imprisonment. Gradually, a mesmerising pattern emerges.