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[rivka, Former Moderator]

Some books exist (either in whole or in part) as HTML or other ebook forms. IMO, there is no reason it should have a link to a specific site selling the book or to the author's page, and I remove those links when I see them.

For most books, the field will not be used.

[Cait]

Sometimes an author's website will have a page specifically for that book; I'll put in those links, if I care enough about the book and the author's book page. :) But general author websites should go on the author's profile, definitely, and I'll take down links to booksellers' websites or wikipedia or the like....

[rivka, Former Moderator]

Sometimes an author's website will have a page specifically for that book; I'll put in those links

That seems quite reasonable. I hadn't seen any like that (except that have part as an ebook, as above), but it makes sense.