This was one of my early favorites by Jack Chalker. Victor Gonser, a 35 year old PHD, is on a backpacking trip in Alaska when he finds himself in the body of a 13 year old Indian girl and later in the body of a 18 year old blond beauty. What happened? With Chalker you can bet there are probably aliens involved, but there's no telling what twists and turns the story may take.