My co-author and I figured out that a positive feedback loop is key. In addition to our API Design Book at http://theapidesignbook.com/ my co-author and I have a weekly newsletter at http://bit.ly/apiWeekly then he followed the consulting route using those for lead generation while I did some classes for Lynda/LinkedIn Learning: https://www.linkedin.com/learning/instructors/keith-casey and in both cases, we refer back to the book and the newsletter. Then in my day job at Okta, I talk about the same topics and occasionally refer to our stuff among other things as "for more information."
Most of it is free, a few things are paid, but everything builds and complements each other.
No matter where anyone finds us, the threads lead to the other things. This allows people to understand the basics of APIs, learn some advanced topics, and generally stay up to date on the space as a whole.