The Prince of Peril (Ace Books, 1963)

Protagonist Robert Grandon swings through a cityscape that is at once futuristic and reminiscent of ancient architecture, in this swashbuckling cover from the master of pulp hero paperbacks: Roy Krenkel! This dynamic image appeared on the paperback edition of the second book in Otis Kline's Peril series. After all, when you're publishing a story by one of Edgar Rice Burroughs' main rivals in the planetary romance and pulp fantasy realm -- and Burroughs' own paperbacks feature mind-blowing Krenkel covers -- what choice did Ace Books have but to follow suit with that winning strategy?