There isn't any UI for this, but it's in this proc:
ad_proc get_book_url {
{-associate_id "pindsdotcom"}
isbn
} {
So you can pass in a different Amazon associate ID.
I didn't bother to make a UI for this, because I figured that you really want to generalize this. What if you happen to prefer Amazon.co.uk, or some other bookshop entirely.