From the web site: "Create a mini-Phoenix framework using Elixir and Cowboy. What can be the best way to understand Phoenix than to create it yourself."
Garuda, I'm sure after googling you found the Wikipedia entry for it. It is just a play on words since Garuda, like Phoenix, is a type of mythical bird.
Tomte, "Garuda" is not a random title. As you and others have guessed it, Garuda is a mythical bird in Indian mythology just like Phoenix being a mythical bird in Greek mythology. Since the book is about "Demystify Phoenix Internals & Rebuild Phoenix Clone", I have named it as Garuda.
I agree with you, I could have explained the book's content and its title a bit more clearer on my website. I will do it sometime soon.
It's a strange way to sell a book, when neither the web site selling the book nor googling seem to enlighten me.