: My guess is that the original use was INTENDED to be for when the company didn't know what the last digit of the ISBN number would be. It could have also have been intended to indicate that this title was going to use up 0 through 9 for the last digit in the book's various reprints.

: In actuallity, there are many books that have the last digit end in an "X" and there are very few books that really seem to have a consistant printing number in that last digit.

If I remember correctly, from when I was half owner of SF&M Books, the X was originally supposed to indicated "We don't know what FORMAT it will be, yet, or something like that, e.g. size or hard vs. soft.

They're also SUPPOSED to change the ISBN in SOME WAY when they change the price. They've gotten a lot better at that. In practice, "they're just flakey" on the one hand, and "the system can't deal with the volume so weird things are done" on the other hand. Complicating factors are that those numbers actually cost the publisher a small amount of money ... and a lot of them don't like the system AT ALL ... but prefer some other code/number system ... but they're stuck with it.