This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I don't have HFSJ, so I can't explain exactly what the book page is saying, but the requirement is just that the generated class implement HttpJspPage. The container is not required to do so by extending HttpServlet, although it certainly could if it wanted to.