This system is exactly how pragprog.com works. You can even purchase a PDF/book combo before the book is completed and have access to a (constantly updated) be

This system is exactly how pragprog.com works. You can even purchase a PDF/book combo before the book is completed and have access to a (constantly updated) beta version of the PDF. When the book is completed, the PDF is finalised and the book itself is mailed to you.