I should preface my comments here by saying that I have not yet read Newsletter Ninja, but I have known the author for several years and I've been able to glean a lot of information from her already. Newsletter Ninja is on my to-be-read list, as I am confident that it will expand on what I've already learned.
When you rely on Amazon or Facebook or any other entity to stay in touch with readers, you are at the mercy of that third party's intentions and decisions. What will you do if they go away, or change their policies abruptly, or kick you out because of some perceived problem? To insulate yourself from potentially-surprising disruptions, it makes sense to have your own way to stay in touch with your fans without a gatekeeper. A highly effective way to do that is to develop a newsletter—not just a list of addresses, but actual content that people are accustomed to seeing in their inbox.
Newsletter Ninja has gained a lot of respect in indie writer circles for providing a solid basis for maximizing the value of your newsletter. Grab your copy today and work on building your audience on your ground.