This idea has arisen before; as Kiwidude said
Originally Posted by kiwidude
Unfortunately Shelfari has no API so because of that and it has very limited integration with anything else which is a big thumbs down - and Amazon now owning it kills any innovation in that area. People have complained about it for years and that hasn't changed. A plugin would have to do it with web scraping, which is slow and fragile, prone to breaking every time they tweak their website.
Not impossible, but not much fun to write/maintain either.