Couple of observations:
- the page size drop down doesn't display any units (e.g. "6 x 9"). I assume there're all in inches but it would be a little more helpful if it said so and/or included a common name (e.g. US Letter) if one exists for that size.
- you might want to look into page imposition[1] something that's basically essential for any kind of stitched binding (as opposed to "perfect binding"). Full-blown imposition software is often ridiculously expensive and can have quite a few options so it's definitely both an engineering and UI challenge. In the meantime, Bookbinder JS[2] is a great site that I think runs entirely client side and can transform any PDF.
1: https://en.wikipedia.org/wiki/Imposition
2: https://momijizukamori.github.io/bookbinder-js/?paperSize=LE...
Ha ha. I never thought of that as a selling point for affiliate links. I suppose Amazon will make less money if people print their own books as well.