List of Ace titles in numeric series
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.