I can understand the dislike of signing up to a service but in this case that's the price of the book. I also dislike signing up to various services, but I did for Road to React and I can confirm I haven't recieved any spam. In fact the first message I recieved was about this new book. I wouldn't worry too much.
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.