Agent of the Terran Empire by Poul Anderson

Paperback. Condition: Good-. 1979. ACE.

Flandry, here a captain, undergoes a series of adventures: He is abducted by the Scothians, an alien race hoping to invade the Terran Empire; rescues the Emperor's kidnapped granddaughter; meets Aycharaych, his nemesis in league with the Merseians; and investigates a previously unencountered alien race that has invaded the distant colonial world of Vixen.

Fifth book with Flandry