It's actually easier to implement as a special page as Brion points out: ISBNs will generate a link to "/wiki/Special:Booksources?isbn=XXXXX", and that special page will have links to Pricescan, AddAll, Amazon,
Why not allow for Special:Booksources?isbn=XXXXX&titlewords=YYYY&author=ZZZ with all values optional. All the above mentioned sites can search books based on author and title. We could then also add ebay links for example, as well as links to catalogs of libaries which participate in interlibrary loan.
The Title/Author combination has the advantage that it captures all editions and therefore doesn't age like ISBN numbers do.
We just have to come up with some catchy wiki notation for the book links. Maybe [[book:Douglas Hofstadter|Godel, Escher, Bach|0465026567]] (again with all fields optional) could turn into a link to Special:Booksources and render as something like "(find this book)".
Axel