If you're an author and you don't have a website for your book you need to read this brief article on Publisher's Weekly. Although it should be clear by now that every book needs a companion website, far too many books still don't have one. Besides that obvious fact, the article also points out that more people tend to go to an author's website than a publisher's website (23% for the former and only 18% for the latter). Let's face it. Publisher websites tend to be nothing more than online catalogs. Not a lot of personality and rarely a chance for a reader to interact directly with an author.
Authors, if you didn't think you needed a website for your book before, I urge you to reconsider now that there are some solid numbers to back it up.