In January 1969, Ace Books switched from a letter-series code for its books to a numeric series. The number does not indicate sequence of publication, unlike the number in the letter series codes; instead it identifies the alphabetic position of the title. It was assigned by dividing the range 00001-99999 into 26 sections, one for each letter of the alphabet, and then assigning the code depending on the first letters of the title. As can be seen from the list below, this approach was evidently not followed in every case, but it accounts for the great majority of the codes. The number is also part of the ISBN, for the later titles; the ISBN for a book (if it has one) can be formed by prefixing "0" for English language/US, and "441" (Ace's publisher number), to the serial number, and then calculating the last digit with an ISBN check digit calculator. For example, Christopher Stasheff's Escape Velocity has serial number 21599; the ISBN is 0-441-21599-8. This list is very incomplete. 00075 SF John Jakes When the Star Kings Die 00078 SF Peter George (as Peter Bryant) Red Alert (1958) 00078 SF R.A. Salvatore The Dragons Dagger 00092 NA John Macklin Dwellers in Darkness 00093 SF Fred Saberhagen The Black Mountains 00094 SF Leigh Brackett The Big Jump 00104 SF Mack Reynolds Section G: United Planets 00106 NA John Macklin Passport to the Unknown 00107 SF James White The Secret Visitors 00108 SF Roger Zelazny Four for Tomorrow 00109 SF Mark S. Geston Lords of the Starship 00110 SF Fritz Leiber Swords in the Mist 00111 SF John W. Campbell Invaders from the Infinite 00119 SF William Shatner Teksecret 00125 SF Mary Staton From the Legend of Biel 00142 SF Steve Perry The Forever Drug (1995) 00153 SF Fritz Leiber Swords Against Wizardry 00241 NA Jack Luzzatto Ace Crossword Puzzle Book#2 (1969) 00265 SF Mack Reynolds Ability Quotient 00275 SF Donald A. Wollheim (ed.) Ace Science Fiction Reader (1971) 00289 SF William Shatner Tekpower 00348 SF Greg Bear Blood Music (1996) 00390 SF William Shatner Tekmoney 00950 SF Ron Goulart After Things Fell Apart (1970) 00958 SF Mack Reynolds After Utopia (1977) 00990 SF Susan K. Putney Against Arcturus / Dean R. Koontz Time Thieves (May 1972) 01000 SF John Brunner Age of Miracles 01040 SF Larry Maddock Agent of T.E.R.R.A.#1: The Flying Saucer Gambit 01041 SF Larry Maddock Agent of