I get the frustration about the email, but that aside, I do wonder whether requiring the email before getting the book is the most effective mechanism for your end goal. What do you want to achieve? Do you want to maximize the number of people subscribed? Do you want to create a community? Why do you want these things? Do you want to share your knowledge? Having no email-wall would probably result in fewer registrati…
Instead, it advertised free, then there is the wall. Like many others, I feel like I've been drawn to a false promise.
I totally understand the author can keep the book behind whatever wall he wants. The title could have been "I wrote a book about ...". And then in the site, "subscribe to get the book".