Congrats! This is a really cool opportunity. I would suggest thinking about what kind of a book it's going to be, and who the audience is. Is it more of a refe

Congrats! This is a really cool opportunity. I would suggest thinking about what kind of a book it's going to be, and who the audience is. Is it more of a reference book for people who know HTML and want a hard copy? Or is it for people who are learning HTML for the first time, and want to learn HTML5? Is it a book that assumes people know what vim and emacs are? Or is it a book that takes 1-2 paragraphs to explain what a text editor is? Is it a book for nerds? Or a book for retired dads looking to make their first website? I'm not going to say 500 pages is too much or too little, because I don't know what kind of a book you're writing yet. Also: some hackers here already mentioned that a lot of HTML5 information is available on the internet. I would think carefully about what your book will do that a google search can't.