Maybe it's not the best book on GraphQL, I don't know, but it is miles better than most paid tech-books out there.
The book is self-published and the author isn't well known, so he needs to something to build an audience. Giving them such a good book for an email is a nice way to do so.
I'm not well known either and my publisher only gives away one chapter for an email and then you have to buy it.
