Piper in the Woods by Philip K. Dick, Science Fiction, Fantasy, Adventure (Paperback) (ISBN-13: 9781463800116)

"Well, Corporal Westerburg," Doctor Henry Harris said gently, "just why do you think you're a plant?" As he spoke, Harris glanced down again at the card on his desk. It was from the Base Commander himself, made out in Cox's heavy scrawl: Doc, this is the lad I told you about. Talk to him and try to find out how he got this delusion. He's from the new Garrison, the new check-station on Asteroid Y-3, and we don't want anything to go wrong there. Especially a silly damn thing like this!