ISBN number registartion

Hello all,

I notice that the R package, the manuals and even the FAQ have ISBN
numbers. I help write the Gnumeric (GNOME's spreadsheet) manual and
wondered how you all managed to get those numbers. Also are these
numbers interrelated so that if you gain a document, its number would
follow from your existing numbers.

Any pointers to information would be appreciated.

Thanks,
--adrian custer