There are a lot of books about sales funnels.
I haven't read many others, but this one is, by far, what truly made me understand online business.
The best book on sales funnels I've found so far is "Dotcom Secrets" by Russell Brunson.
This book's best-kept secret:
The book introduces the concept of the "value ladder." This is the #1 thing that will turn your followers into high-paying customers. You business is supposed to bring them into your world by offering them a "freebie." Once they opt-in, you can then nurture them via email and introduce them to your paid offerings.
If you are at all interested in sales funnels, I can't recommend this book enough.
(And if you've read it, do you agree? What other book would you recommend for beginners?)