marketing - Should an author have one website or two?

I have seen that sometimes an author will have one website with a lot of content and a separate website that does nothing but offer the book(s) for sale. What are the advantages of this approach? What are the advantages of having only one website, and offering the book for sale directly on that website (which would have supporting articles, perhaps some unrelated content, perhaps some interactive features, more detail about the author, and perhaps some sections from the book(s) to whet the reader's appetite)?