Your nonfiction email list subscribers may have signed up when you offered a valuable free report. Perhaps you shared tips from your book. To keep your email list subscribers you'll need to continue offering valuable content with each email you send. When you do this your readers will come to know that you are an authority in your field. It's you they'll turn to when they need information on the topic you're an authority on.
Don't hold back thinking you'll save it for your next book. You can still include it in the book, perhaps expanded with more details and statistics. In fact, you'll be more likely to sell more books to your list because they will have a taste of what's in it. As with any other marketing, remember the 80/20 rule and provide 80% content to 20% book marketing in your emails. … more … "Writing for Your Nonfiction Email List"