Like many of the stories in this section, the surprise isn't in the finding of the treasure, it's in the bookseller "Doing the right thing" and returning the jewelry to it's "rightful" owner.
Think, then, of how often this occurs (and everyone knows the hollowed out book trick); and yet how seldom we hear about it. This is probably up there with the "reported" treasures found by builders when doing renovations on homes (practically nonexistent, yet rest assured there's a large underground traffic in antiquities and art recovered from properties where the owner is never made aware of it's existence).
Link: http://www.odt.co.nz/news/dunedin/143459/jewellery-stash-first-secondhand-book-seller