This is a short novel I remember from a long time ago. One of those thin sci fi novels popular in the 70s and 80s.It was a space opera story. The story revolves around a spy/rogue that is on vacation or leave on a pleasure planet. He is given an assignment during the vacation to rescue a girl from a band of kidnappers/slave traders. I only read the first chapter or two. So thats about all I remember of the book. However I remember that this ones cover (was a paper back), had a picture of two "frogmen" amphibian aliens in clothes and with rifles on the cover. Anyone have an idea of the title or author of this book?