Andy,
The /book/book_edition/ISBN is a mix of -13 and -10 sometimes even weirder :)
It was deprecated (but still holds value) about 3 years ago by Jeff Prucher @ Google and we moved the -13 instances into
Although I do not now if the copy (done by a bot script at the time by Metaweb) was complete. It seems to be however by quick glance... there are 1.6 Million Cataloged instances alone from various OPACs, etc. that were imported into Freebase in 2012.
I should probably jump into this and help you guys with this mapping.
Thad