I've added the link in the back of my book inviting people to join but it hasn't been very effective.
Offering people something of value tends to work much better. When they see a request to join your mailing list there is very little incentive for them to click on the link you've provided and do you a favor.
Something that works much better is creating a "funnel" that draws those bees(readers) to the honey (your free book).
And the best way to do this is to make one of your books permanently free(this always works best if it is the first book in a series) so you can broaden your reach. Most people are willing to take a chance on an author they have never heard of if they aren't risking any money in the process.
Some authors absolutely freak out when I tell them to do this. Believe me, I feel your pain. You worked like crazy on that sucker, and giving it away for free makes you feel like you're internally bleeding.
But any good business knows that giving samples of their products or services is the best way to create loyal clients who will continue to buy their products. The business side of being an author is no different.
Give them a generous sample of your work, and make it high quality. This free book is either going to be the thing that makes them stay or makes them wander to someone else's book.
So that perma-free book is your funnel book. Once you have that perma-free book set up, create a graphic of an entirely different book, possibly the second book in a series, and stick that sucker at the beginning of your book with a Call To Action just underneath it. Something like this.
Download Your Free Book Today!
Once people click on that link to get their free book, it should take them to a leadpage or landing page that allows them to grab your book and allows you to collect their email address.
Be sure to place that graphic in the front and back of your book, and make sure you get that funnel book posted on as many platforms as possible so you can keep drawing those bees to that honey.
If you're still feeling a little unsure about the process, just set up a call with me and I'll walk you through it.
Excited to work with you.