The distribution model is super dodgy. You have to connect to Facebook messenger and then also give an email address which is never actually used in the process of sending you the book.
EDIT: It also opts you in to the messenger subscription, in direct breach of the Facebook usage guidelines. But, I guess, this is a "growth hack".
EDIT2: I have absolutely no problem with asking for a messenger or email subscription, that is something completely normal and acceptable in marketing and is a great way for people who are interested to stay engaged with your brand.
The difference here is that you do not ask for a messenger or email subscription. You tell people they can get the book by pressing that button, but don't tell them that it will also opt them in to a messenger subscription. And then you ask for an email to which you will send the book, but then don't actually send the book to that email. I assume you add that email to your mailing list given what you've said here.
This is absolutely misleading. The user does not knowingly opt-in to these subscriptions, and you aren't told how to opt-out unless you read the book (yes, I did read it!).
A simple disclaimer telling the user that by clicking the link they are subscribing to a messenger list would completely fix this.
