and have it just work,  but because a number is never valid in the QName,  you can encode the URI like this:

http://isbn.example.com/I0884049582

and then write

isbn:I0884049582

<I0884049582>

which is not too bad.  Note,  however,  if you want to write

<0884049582> you have to encode as

http://isbn.example.com/I0884049582

because,  at least with the Jena framework,  the same thing happens if you write

@base <http://isbn.example.com/I>