Ace Books have published hundreds of science fiction titles, starting in 1953. Many of these were Ace Doubles (dos-a-dos format), but they also published many single volumes. Between 1953 and 1968, the books had a letter-series identifier; after that date they were given five-digit numeric serial numbers. There were a total of 378 letter-series sf titles (62 S&D, 174 F, 19 M, 78 G, 29 H, 1 N, 1 K, and 14 A series books). The list given here gives a date of publication; in all cases this refers to the date of publication by Ace, and not the date of original publication of the novels. For more information about the history of these titles, see Ace Books, which includes a discussion of the serial numbering conventions used and an explanation of the letter-code system. D and S Series S-066 SF L. Ron Hubbard Return to Tomorrow (1954) S-090 SF Robert Moore Williams The Chaos Fighters (1955) D-096 SF Andre Norton The Last Planet (special edition – half of the Ace Double with same code – 1955) D-110 SF Isaac Asimov The 1,000 Year Plan (special edition – half of the Ace Double with same code – 1955) D-121 SF Andre Norton The Stars Are Ours! (special edition – half of the Ace Double with same code – 1955) D-125 SF Isaac Asimov The Man Who Upset the Universe (1955) S-133 SF Donald A. Wollheim (Ed.) Adventures On Other Planets D-146 SF Murray Leinster The Forgotten Planet (special edition – half of the Ace Double with same code – 1956) D-155 SF Jules Verne A Journey To The Center Of The Earth (1956) D-169 SF Jack Williamson And James E. Gunn Star Bridge S-183 SF Donald A. Wollheim (Ed.) The End of the World D-187 SF A. E. van Vogt The Pawns of Null-A D-211 SF Philip K. Dick Eye in the Sky (1957) D-233 SF Rex Gordon First on Mars D-245 SF Jules Verne Off on a Comet (1957) D-261 SF Philip K. Dick The Variable Man And Other Stories (1957) D-274 SF Charles Eric Maine World Without Men (1958) D-283 SF Clifford D. Simak City (1958) D-309 SF H. G. Wells The Island Of Dr. Moreau (1958) D-324 SF Ray Cummings Brigands of the Moon (1958) D-327 SF Jefferson Howard Sutton First on the Moon D-339 SF Clifford D. Simak Ring Around the Sun D-340 SF Philip K.