I probably wouldn't buy a book - I use
W3Schools Online Web Tutorials for my HTML questions.
However, if you believe you have a great idea, choose a publisher that would be a good fit for an HTML5 book and send them a query. You can probably find the type of information they want in a query from their site. Tell them what you want to do and why you think it's important.
Another opportunity is magazines. You could do as Ahmad suggested and describe the new features of HTML5.
You could create a PDF to share your HTML5 ideas, and sell it online.
Good luck!