Title: Trader to the stars
Description: From inside front jacket:

TRADER TO THE STARS: Van Rijn boards a ship in space and discovers that he's captured a zoo.  The race that had commanded the ship have hidden themselves among the other animals... and Nicholas, unable to operate the ship's controls, must determine which of the animals *can*.

TERRITORY:  Nicholas and a pretty young biotechnician are attacked by the natives of an ammonia-enveloped planet.  Their only hope of escape lies in turning one faction against another... and then uniting them to meet still another.

THE MASTER KEY: Van Rijn plays Sherlock Holmes as he lounges in his penthouse and hears a baffling tale of interstellar double-cross.
Subjects: Fiction in English, American Science fiction, Science fiction, American