Agent of the Terran Empire, by Poul Anderson

The best part of Agent of the Terran Empire is the afterward by Sandra Miesel.

Tiger by the Tail: Flandry is kidnapped by the Scothani, an upstart empire with ambitions of ruling the galaxy; Flandry manipulates the Scothani factions and turns them against each other before they can mount an effective strike against the Terran Empire.
The Warriors From Nowhere: An unidentified raiding party attacks Fort Lone on Varrack, and kidnaps Her Highness Lady Morgan, granddaughter of the Emperor. The clues point to Merseia, but Flandry suspects Duke Alfred of Tauria.
Honorable Enemies: Flandry meets his match in Aycharaych, an alien sapient who can read minds. To defeat him, Flandry's consort Aline tricks him into believing the Empire is planning to occupy Betelgeuse, goading Aycharaych into taking premature action.
Hunters of the Sky Cave: A mysterious fleet besieges the planet Vixen. The obvious clues implicate the Ymirites, although the hydrogen-breathers have no reason to concern themselves with oxygen-based races. Flandry travels to Vixen, runs the blockade, and works to infiltrate the alien organization and discover who is behind the well-timed attack on the Empire.
Lurex and Gold: Poul Anderson's Dominic Flandry Series, by Sandra Miesel: A broad overview of one of the greatest SF series of all time.