In 1952 Wollheim left Avon to work for A. A. Wyn at the Ace Magazine Company and spearheaded a new paperback book list, Ace Books. In 1953 he introduced science fiction to the Ace lineup, and for 20 years as editor-in-chief was responsible for their multi-genre list and, most important to him, their renowned sf list. Wollheim invented the "Ace Doubles" series which consisted of pairs of books, usually by different authors, bound back-to-back with two "front" covers. Because these paired books had to fit a fixed total page length, one or both were usually abridged to fit, and Wollheim often made other editorial alterations—as witness the differences between Poul Anderson's Ace novel "War of the Wing-Men" and its definitive revised edition, "The Man Who Counts". Among the authors who made their paperback debuts in Ace Doubles were Philip K. Dick, Samuel R. Delany, Leigh Brackett, Ursula K. Le Guin, and John Brunner. William S. Burroughs' first book, "Junkie", was published as an Ace Double. Wollheim also helped develop Marion Zimmer Bradley, Robert Silverberg, Avram Davidson, Fritz Leiber, Andre Norton, Thomas Burnett Swann, Jack Vance, and Roger Zelazny, among others. While at Ace, he and co-editor Terry Carr began an annual anthology series, "The World's Best Science Fiction", the first collection of what they considered the best of the prior year's short stories, from magazines, hardcovers, paperback collections and other anthologies.