Especially since the official site of a book is usuallly not suitable for that (no forum or separate login per publisher etc.)
Or in the case of Pragmatic Programmers not accessible, if you click on discussion forum at https://pragprog.com/book/swdddf/domain-modeling-made-functi... you get redirected to: https://forums.pragprog.com/fosta-sesta
Anyone else who is missing something like this?
Edit: looks like it's just for reviews.
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.