Bladesman of Antares. Dray Prescot, #9

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 flags, his mission, and his life, is perilously balanced upon the razor-edged blades of loyalty versus duty... Bladesman of Antares is the ninth book in the epic fifty-two book saga of Dray Prescot of Earth and of Kregen by Kenneth Bulmer writing as Alan Burt Akers. The story follows Fliers of Antares, and is continued in the next book in the series: Avenger of Antares.