Bladesman of Antares — Alan Burt Akers

The ninth book of the Dray Prescot series.The problem with being a spy is that you have to make friends with the enemy. Dray Prescot, Earthman, who had become prince of Vallia, was the kind of man who always stood by his friends. So in his quest to learn the war secrets of Hamal, empire of the aircraft-makers, Dray found himself not only becoming comrade to some of its greatest warriors but of championing the very life of its cruelly beautiful queen. Although Dray's devotion to his glorious princess Delia never flagged, his mission, and his life, was perilously balanced upon the razor-edged blades of loyalty versus duty...