I didn't put my email to get the book. I would probably have subscribed, if it was something like "if you enjoyed the content, do subscribe to get more".
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".
HN should have a tag for walled contents.