Beholder's Eye (Web Shifters) by Julie E. Czerneda ISBN 13: 9780886778187

They are the last survivors of their race, beings who live on and communicate through energy, who are capable of assuming the shape of any other species. When their youngest member is assigned to a world considered safe to explore, she is captured by the natives. To escape, she must violate the most important rule of her kind, and reveal the existence of her species to a fellow prisoner--a human being. Now her race is in danger of extinction, for even if the human does not betray her, the Enemy who has long searched for her people may finally discover their location....