Capps, Carroll M. (=MacApp)
SECRET OF THE SUNLESS WORLD
Carroll Mather Capps wrote 30+ stories (1960-72) and 7 novels (1968-72): mostly under the pseudonym "C. C. MacApp." 1 story (1967) and 1 novel (1969) appeared as by Carroll M. Capps.
Shann was a world without a sun. It was livable, but nearly impossible to find - a perfect rendezvous for pirates and outlaws, and a good place to hide secrets. Colonel Vince Cullow was given a mission: stop the Chullwei before their ruthless empire-building overran the galaxy. He had to start as prisoner of the pirate Gondal - who wanted the secret of the humanoids who had strangely vanished after reigning over all of space, and thought the secret was on Shann.