Frank Belknap Long 
Space Station 1 

The Space Station floated up out of the Big Dark. Lieutenant Corriston had come to see its marvels, but he soon found himself trapped in unexpected terrors. The grim reality was that an unknown, unsuspected outer space power had usurped control of the artificial moon. A beautiful woman had disappeared; passengers were being fleeced and enslaved; and, using fantastic disguises, imposters from – SOMEWHERE – were using the station for their own mysterious ends. (Goodreads)