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!