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 T.E.R.R.A.#2: The Golden Goddess Gambit 01042 SF Larry Maddock Agent of T.E.R.R.A.#3: The Emerald Elephant Gambit 01043 SF Larry Maddock Agent of T.E.R.R.A.#4: The Time Trap Gambit 01066 SF Poul Anderson Agent of the Terran Empire 01501 SF Robert A. Heinlein The Worlds of Robert A. Heinlein 01570 SF Fletcher Pratt Alien Planet 01625 NA Dick Lupoff and Don Thompson (ed.) All in Color for a Dime (1970) 01685 SF Gordon R. Dickson Alien Art / Arcturus Landing 01750 SF Robert E. Howard Almuric 01770 SF Leigh Brackett Alpha Centauri Or Die (1976) 02236 SF Stanley Schmidt (ed.) Analog Yearbook Ii 02255 SF A. E. van Vogt Anarchistic Colossus (April 1977) 02268 SF Joanna Russ And Chaos Died (1970) 02274 SF Donald R. Bensen And Having Writ... (1978) 02276 MY Philip Loraine The Angel of Death (1961) 02295 SF Keith Roberts Anita (1970) 02320 SF Alexei Panshin Masque World 02380 SF Tim Powers The Anubis Gates 02900 NA John Jakes (as Jay Scotland) Arena 02935 SF Philip Francis Nowlan Armageddon 2419 A.D. 02936 SF Philip Francis Nowlan Armageddon 2419 A.D. 02938 SF Philip Francis Nowlan Armageddon 2419 A.D. 02940 NA Rona Randall The Arrogant Duke (1972) 03297 SF Jack Vance The Asutra 03300 SF John Brunner The Atlantic Abomination (1960) 03322 SF Edgar Rice Burroughs At the Earth's Core 03325 SF Edgar Rice Burroughs At the Earth's Core 03326 SF Edgar Rice Burroughs At the Earth's Core (1978) 03328 SF Edgar Rice Burroughs At the Earth's Core (1985) 04040 SF Joanna Russ Picnic on Paradise 04591 SF Samuel R. Delany Babel-17 (1966) 04592 SF Samuel R. Delany Babel-17 (1974) 04612 WE Tom West Bad Blood at Bonita Basin / Tom West Rattlesnake Range (1972) 04636 SF Edgar Rice Burroughs Back to the Stone Age 04722 SF Samuel R. Delany The Ballad of Beta 2 04745 NA Edgar Rice Burroughs The Bandit of Hell's Bend 04745 WE Edgar Rice Burroughs The Bandit of Hell's Bend 04755 NA E. Kelton Barbed Wire 04760 SF Tom Purdom The Barons of Behavior (1972) 04860 SF A. E. van Vogt The Battle of Forever (1971) 05330 SF Jack London Before Adam 05404 SF Fred Saberhagen Berserker 05407 SF Fred Saberhagen Berserker Man (April 1979) 05408 SF Fred Saberhagen Berserker's Planet (1980) 05424 SF Fred Saberhagen Berserker Man (December 1980) 05454 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 15th Series (1966) 05455 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 16th Series 05456 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 17th Series 05457 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 18th Series 05458 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 19th Series (1973) 05460 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction: A Special 25th Anniversary Anthology 05461 SF Edward L. Ferman (ed.) The Best from Fantasy and Science Fiction, 22nd Series (1978) 05466 SF Fred Saberhagen Berserker Man (August 1982) 05471 SF Fred Saberhagen Berserker Man (May 1984) 05475 SF Lester Del Rey (ed.) Best Science Fiction Stories of the Year (1st Annual Collection) (1972) 05476 SF Lester Del Rey (ed.) Best Science Fiction Stories of the Year (2nd Annual Collection) 05477 SF Lester Del Rey (ed.) Best Science Fiction Stories of the Year (3rd Annual Collection) (1974) 05478 SF Lester Del Rey (ed.) Best Science Fiction Stories of the Year (4th Annual Collection) (1977) 05479 SF Lester Del Rey (ed.) Best Science Fiction Stories of the Year (5th Annual Collection) (1977) 05481 SF Mack Reynolds The Best Ye Breed 05496 SF Fred Saberhagen Berserker Man 05500 SF Robert A. Heinlein Between Planets 05586 SF John Varley The Golden Globe 05595 SF John Rackham Beyond Capella / Kenneth Bulmer Electric Sword-Swallowers (February 1971) 05655 SF Edgar Rice Burroughs Beyond the Farthest Star (1973) 05656 SF Edgar Rice Burroughs Beyond the Farthest Star 05785 NA Shepherd Mead The Big Ball of Wax 06061 SF Leigh Brackett The Big Jump (1976) 06171 SF Jack Vance Big Planet (1978) 06177 SF Keith Laumer The Big Show 06505 MY Cornell Woolrich The Black Angel (1965) 06530 SF Michael Moorcock The Black Corridor 06612 SF Mack Reynolds Blackman's Burden / Border, Breed Nor Birth (August 1972) 06615 SF Fred Saberhagen The Black Mountains 06701 SF John W. Campbell The Black Star Passes 06707 SF Brian M. Stableford The Blind Worm / Emil Petaja Seed of the Dreamers (February 1970) 06715 NA Charles Lefebure The Blood Cults (1969) 06760 WE Ray Hogan The Bloodrock Valley War / C. Hall Thompson The Killing of Hallie James (1969) 06854 SF Marion Zimmer Bradley The Bloody Sun 07012 SF L. Sprague de Camp and Catherine Crook De Camp The Bones of Zora 07080 NA Joyce Keener Border-Line (1979) 07162 SF John Brunner Born Under Mars 07180 SF Marion Zimmer Bradley The Brass Dragon 07200 SF Jack Vance The Brave Free Men: Book II of the Durdane Trilogy 07690 SF Murray Leinster The Brain-Stealers 07840 SF Ray Cummings A Brand New World 07895 SF Andre Norton Breed To Come (1973) 07921 MY Cornell Woolrich The Bride Wore Black 08145 SF John Rankine The Bromius Phenomenon 08215 SF Fred Saberhagen Brother Assassin 08560 WE Tom West The Buzzard's Nest / Louis Trimble Siege at High Meadow (1973) 09022 SF Robert O'Riodan Cadre One 09037 SF Spider Robinson Callahan's Crosstime Saloon (1977) 09069 SF Spider Robinson Callahan's Crosstime Saloon 09072 SF Spider Robinson Callahan's Lady (1989) 09102 NA Nancy Buckingham Call of Glengarron 09128 SF Kenneth Von Gunden K-9 Corps 09135 WE Sam Bowie Canyon War / Clay Ringold The Hooded Gun (1969) 09200 SF Edgar Rice Burroughs Carson of Venus 09203 SF Edgar Rice Burroughs Carson of Venus 09205 SF Edgar Rice Burroughs Carson of Venus (1982) 09265 SF Andre Norton Catseye 09281 SF Edgar Rice Burroughs The Cave Girl 09284 SF Edgar Rice Burroughs The Cave Girl 10150 SF Walt Richmond Challenge the Hellmaker 10258 SF Margaret St. Clair Change the Sky and Other Stories (1974) 10293 SF Ken Bulmer The Chariots of Ra / John Rackham Earthstrings (July 1972) 10307 SF Ursula K. Le Guin City of Illusions (1967) 10410 SF A. E. van Vogt Children of Tomorrow 10411 SF A. E. van Vogt Children of Tomorrow 10471 NA Sam Bowie Chisum (1970) 10600 SF Robert A. Heinlein Citizen of the Galaxy 10621 SF Clifford D. Simak City 10665 WE Giles A. Lutz The Challenger / Tom West The Phantom Pistoleer (1960) 10701 SF Ursula K. Le Guin City of Illusions (1967) 10702 SF Ursula K. Le Guin City of Illusions 11036 SF Philip K. Dick Clans of the Alphane Moon (1972) 11182 SF Ron Goulart Clockwork Pirates / Ghost Breaker (March 1971) 11222 NA Nancy Buckingham Cloud Over Malverton 11451 SF A. Bertram Chandler Coils of Time / Into the Alternate Universe (November 1972) 11457 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan the Freebooter 11467 SF Robert E. Howard, Björn Nyberg, and L. Sprague de Camp Conan the Avenger (Conan #10) 11530 WE Dwight Bennett Newton (as Clement Hardin) Colt Wages / Louis Trimble The Lonesome Mountains (1970) 11546 SF Leigh Brackett The Coming of the Terrans (1976) 11555 SF A. Bertram Chandler The Commodore at Sea / Spartan Planet 11556 SF A. Bertram Chandler The Commodore at Sea / Spartan Planet (same cover as 11555-1) 11560 SF Suzette Haden Elgin The Communipaths / Louis Trimble The Noblest Experiment in the Galaxy (October 1970) 11603 SF Robert E. Howard (edited by L. Sprague de Camp) Conan the Conqueror (Conan #9) 11622 SF Anthony Boucher The Complete Werewolf & Other Stories of Fantasy & Science Fiction (1969) 11630 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan 11633 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan the Wanderer 11650 SF Mack Reynolds Computer War / Code Duello (February 1973) 11659 SF Andrew J. Offutt Conan the Mercenary (1981) 11669 SF L. Sprague de Camp (ed.) The Spell of Conan (1980) 11670 SF L. Sprague de Camp (ed.) The Blade of Conan (1979) 11671 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan (1967) 11672 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan of Cimmeria 11673 SF Robert Howard, L. Sprague de Camp, and Lin Carter Conan the Freebooter 11674 SF Robert E. Howard, L. Sprague de Camp, and Lin Carter Conan the Wanderer 11675 SF Robert E. Howard and L. Sprague de Camp Conan the Adventurer 11676 SF L. Sprague de Camp and Lin Carter Conan the Buccaneer 11677 SF Robert E. Howard and L. Sprague de Camp Conan the Warrior 11678 SF Robert E. Howard and L. Sprague de Camp Conan the Usurper 11679 SF Robert E. Howard (ed. L. Sprague de Camp) Conan the Conqueror 11680 SF Robert E. Howard, Björn Nyberg, and L. Sprague de Camp Conan the Avenger 11681 SF L. Sprague de Camp and Lin Carter Conan of the Isles 11682 SF L. Sprague de Camp and Lin Carter Conan of Aquilonia 11684 SF Andrew J. Offutt Conan and the Sorcerer (1979) 11705 SF Robert Silverberg Conquerors from the Darkness / Master of Life and Death 11738 WE Tom West Corral This Killer / Dan J. Stevens Hunter's Moon (1973) 11759 SF H. Beam Piper The Cosmic Computer 11785 WE Phillip Ketchum The Cougar Basin War / Louis Trimble Trouble Valley 11863 SF Robert E. Howard and L. Sprague de Camp Conan the Freebooter 12126 SF Philip K. Dick The Crack in Space (1966) 12140 SF Brian M. Stableford Cradle of the Sun / Ken Bulmer The Wizards of Senchuria (August 1969) 12311 SF Andre Norton The Crossroads of Time 12313 SF Andre Norton The Crossroads of Time (1978) 13245 SF Alan Dean Foster Cyber Way