The old ISBN10 standard could get X (meaning 10) as the last character (the last number is a check digit), but ISBN13 (which prefixes 978 or 979 to ) is all numbers. The check digit is calculated differently. One obvious advantage is that the ISBN13 is barcode compatible.

I would guess that most modern actors in the book business has been primarily using ISBN13 for at least the last decade.

X can be a check digit. From memory the check digit is calculated using a modulo 11 calculation.