A printed book is a dead end for a reader who wants to act. They finish the last page, think about leaving a review or finding the sequel, then have to type a search into a phone. A QR code removes that gap, which is why the useful placements are the ones where a reader has just decided to do something.
This generator has two modes. One encodes any URL you give it. The other builds Amazon links for you from an ASIN and a marketplace, assembling the product page, the review page and the author page addresses so you do not have to construct them or risk a typo in a link that will be printed thousands of times.
The codes are drawn at medium error correction, a fixed setting rather than a choice. That level rebuilds a code from a partial read, so a small scuff or a crease still scans, at the cost of spending part of the pattern on redundancy — which is why long URLs produce visibly denser grids.
No, it is fixed at the medium setting — a deliberate middle ground with enough redundancy that a scuffed print still reads, without making the pattern needlessly dense. It does mean a very long URL has nowhere to economise, which is another reason to encode a short address.
It assembles Amazon addresses from the identifier and marketplace you supply, giving you a link to the product page, the review page or an author page without writing the URL by hand. The value is accuracy: a single wrong character in a printed link is unfixable.
Large enough that the smallest squares survive the printing process and the scanning distance, which depends on your printer, the paper and the code's density. Rather than trusting any single figure, print it at the intended size on the intended stock and scan it from arm's length.