I think it is good that you have interviewed people that you know and I definitely see the branding appeal of having some big names around. It is just some of the wording in the book could be better, especially since the topic is productivity (not a career success where the argument is definitely stronger).

I am not selling the book yet, I have just a sign up page for now (https://efficientdeveloper.com). My take will be more about software development as a whole rather than just the productivity part, with productivity being just an angle of what I will cover in the book.