All your users registered with gmail will be informed about your new book, and provided 5 alternatives advertised by google.

Maybe they'll even display ads you're paying for in your own email.

It will be crippling for businesses to allow gmail addresses long-term.

Even something as simple as creating an account on an ecommerce site: you register, you get the standard "confirmation email" and google will present you with a whole pile of alternative sites.

Businesses run the risk of losing their customers simply because there will always be a cheaper alternative, and those alternatives will be neatly provided by google on every bit of correspondence you send a gmail user.

Thanks,
Ben