I have an extensive library of books about plants and recently had occasion to quote a section from a book in an answer. As the content of the book is not available online I referred to a listing on Amazon.
On Stack Overflow and all Stack Exchange sites this policy is now in effect which, I believe, indicates attribution for all quoted works in is order.
On this site "attribution" in my answers is usually a link.
When you cannot link to an online source is a top hit like the source on Amazon acceptable?