This site would be both an early access program and an eBook rolled into one - but completely designed to encourage and produce more book sales. Here's sort of how I'd imagine it working:
Anyone who purchases a book (Amazon, in stores, etc.) and sends in a picture of themselves with it gets free access to the site.
Anyone can purchase access to the site, only, for a reduced cost (say, if the book was $40, then the site-only could be $20) and upgrade at any future time (+$20) and get a copy of the book.
Users could purchase access to the site immediately and become part of the editorial process, helping to sculpt the book as it grows. This has been successfully implemented before (the best example being the Django book) and I would like to continue it here.
Lots of new, original, content is released to complement the book. This would include material that wasn't completely appropriate for immediate inclusion (such as working with text ranges or building a WYSIWYG editor) but are valuable nonetheless. Additionally, community-centric parts like a forum could be included for additional discussion.
Occasionally free bits are released (chapter parts, code, etc.) to build excitement and interest for the site and book.