But as for the comment above yours, that is Goodreads. It doesn't sell the books, but is a central place to discuss books. Unlike Amazon.com comments, you don't need to verify anything to be able to comment on a book.
And lastly, it's kind of difficult to say that it's an independent source of reviews, considering that Amazon purchased GoodReads back in 2013, GoodReads offers Amazon-only discounts, and Amazon is always the preferred seller (while the rest of the sellers are hidden behind a dropdown).
LibraryThing has that feature, although the algorithm is not super smart. When I first created an account, I put in all the books I could remember that I had read. Then I clicked on the link to find other users most like me. The top one had a whole series of books that I had read and completely forgotten to add to my catalog. It was uncanny - I couldn't think of any heuristic - they were all mystery novels, and I had almost none in my collection.
Edit: looks like it's just for reviews.