ISBN is the default ID when it comes to book related projects, yes it is convenient but not without its caveats. The often overlooked fact is ISBN was introduced in late 1960s, so books published prior to that obviously does not have that number; and not all countries adopted ISBN from day one, some like China was on its own catalog systems until 1980s; and bc ISBN are usually centralized managed by govt or commercial agencies, censorship with political or commercial reasons are not uncommon, some books were not able to get published, or may only see the world without an ISBN.

For obvious reasons, older / non-English / suppressed books may be those need more care when it comes to preserving.