Phantom of the Seven Stars, by Ray Cummings (epub/Kindle)
Lovely Brenda Carson, scholarly Jerome, pompous Livingston...everyone aboard the Seven Stars scoffed at the idea of a Phantom Pirate. But I.P. agent Jim Fanning didn't laugh. He knew the luxury-liner's innocent looking cargo was already marked for plunder.
Vintage space opera, originally published in Planet Stories, Winter 1940.