I have said elsewhere that you need more than just a book idea to develop a platform. You need the book, of course, but you also need those other things that make you an expert.
As it happens, my platform developed because of other books I have written. I was fortunate enough with my first book to be asked to participate in a project and it sort of went from there. However, that only got me going. Keeping a platform afloat requires real effort.
BestOnlineHighSchools.com is a directory for online high school programs. Over there I also have a forum and a blog that I attempt to update regularly to varying success. In my particular area, I think it is important to offer a service to the public and that is mine. It also generates book sales and Google Adsense money, both very nice things.
What do you offer your book-buying public?