On Jane Friedman site:
Will and Grace, The Gilmore Girls, X-Files, and the upcoming Murphy Brown: Reboots are popular, and we need or enjoy a reboot from time to time. Books are no different.
Book relaunches can take a variety of forms. If done right, they enhance your overall brand, as well as your book sales. Their first and foremost benefit is the new publication date. Having a new book opens up access to bloggers and media who might not have been available to you with an older book. Unless you’ve already been getting some interest in the book, books six months or older are harder to work with. You need to have a new book, or newer book, to capture more blogger and media attention.
Let’s look at some of the reasons you may want to republish your book, because having specific goals is key. Wanting to sell more books is understandable, but it can’t be the only driver—it’s too broad.