List of people who work with Terraform very actively and can tell you a lot (if you ask them) - https://twitter.com/antonbabenko/lists/terraform-experts
There are a lot of Terraform experts who are writing blog posts about Terraform, advices, best practices, open-source projects and other things closely related to the topics described in this book, and I can't think of what can be the best structure to get these links listed here (please leave your feedback).