I've added the link in the back of my book inviting people to join but it hasn't been very effective.
What value do THEY get from subscribing to your newsletter? The answer should be: "lots of value, and I make it obvious to readers in language they care about."
I've written two how-to books. In each, I point to a book-specific landing page with resources. For my first (on public speaking as a form of business development), I describe the resources as templates that will save them 20-50 hours of work. In my second book (on managing marketing & creative teams), it's how the templates will help them become a better manager.
Throughout each book, I have relevant Call to Action (CTA) blocks that point to the resources—an easy "sell" for how-to non-fiction.
If you write fiction, think about what would be compelling. Perhaps your subscribers get first updates about your next book, and private updates about your progress on the writing process (similar to what some Kickstarter campaigns will do).
Make it about value to them, and signups will follow. I'm glad to do a call to discuss further. Good luck!
If you want them to subscribe, you can ask them to mail your the feedback and comments about the book. Take them to a subscription link when they send you mail, in the reply along with thanks. Interested visitors will definitely subscribe to your newsletter.
If you want a growth hacking solution, you may setup a call with me.
Most successful authors make references to their website, especially for worksheets, checklists etc that form an integral part of the message from the book, at several spots within their book.
Once your underlying message is tied in to these resources, the users will feel compelled to visit your website to download this material.
Ask for their email address before they download such material, you'll easily get their subscription.