ABANDONED. Title may be "One"; Sci-Fi; 1960's

[Geneva]

I've been asked to track down a book for a patron.He thinks the title was "One".It came out in the 1960's.It was Sci-Fi. He can't remember the author.He said it's about a man who crash landed on a planet filled with strange creatures who could change their appearance depending on the food they ate.He eventually found a way in to the planet and there he met the aliens who ran the planet. Sound familiar to anyone????

[Lobstergirl, au gratin]

Geneva left the group. Moving to Abandoned.