The Fenris Device: Hooded Swan, Book Five

In a galactic culture that extends from quasi-Utopian worlds like New Alexandria to vermin-infested slums like Old Earth, the Star-Pilots that link the cosmos are the great heroes of the day, and Grainger has become a legend in his own lifetime. The atmosphere of Mormyr is so dense that the surface is unreachable, which made it the ideal place for the alien Gallacellans to hide an ancient spaceship. Now they want it back. Grainger refuses, but when the Hooded Swan answers a mayday call, Grainger and his crew are trapped by a madman, who forces them to pursue the salvage anyway. Success will mean Grainger's freedom from Titus Charlot, owner of the Hooded Swan; failure will mean the death of the inhabitants of an entire planet. And then the Gallacellans appear! Hooded Swan, Book Five.