If you look at the forums for the different series, you'll notice that I have created threads for each new book.  Each thread has a link to the book's page in the bibliography as well as a link for the user to create a review for that book.  The current book on sale plus the next book due to come out will always be pinned at the top of that forum.  Once a new book thread is created, I will un-pin the older book and let it fall naturally.  My hope is that this will encourage some actual discussion about the books, and draw a few more people out to post.