Currently, the only way to email readers, is to publish a new version of a book or course, and when you do that, to select the option to email readers of the book or course.
We recommend people only use that feature when they have added significant new content to a book or course.
Also, please note that readers may have opted-in to share their email address with you. In that case, as long as you explain how you got their email, it is ok to email them.
Here is where you can find these readers for each book: