The Planet Savers (1962) 101 pages by Marion Zimmer Bradley.

Now here's a Darkover novel that I can see inspiring praise for the series. The first two that I read were good, but at times hard to follow.

In this book Dr. Jay Allison has a dual personality. He is a Terran doctor trying to avert a plague on Darkover that happens once every 48 years. Jason's father's plane crashed on a photographic mapping mission, killing the father and leaving 5-year old Jason stranded with the trailmen, who raised him for the next eight years, until his alien-ness was too great.

At age 22 his mountaineering, outgoing personality was subverted and he studied to become a doctor. Now, twelve years later, he is the one that can go to the trailmen and ask them to become volunteers to help the doctors create a serum to fight the 48-year plague.

The heart of the story is the interaction of the characters that have to make the difficult trek into trailmen territory.

Excellent.