As an author, you already are supporting Leanpub by writing and publishing with us.
However, if you are one of our long time authors who has just read our recent "A New Course for Leanpub" essay (https://leanpub.com/anewcourse) and you wish to do something specific, there are two things you can consider doing:
Make a course out of one of your books. Just make a git branch of one of your books, add some quizzes and exercises to it, set it as the repo for a new course and click publish. See https://leanpub.com/markua/read#leanpub-auto-creating-a-course-or-mooc-from-a-markua-document for the Markua spec for making courses, and see https://leanpub.com/courses/leanpub/sample-course for an example course.
Buy a shelf spot for one or more of your books at https://leanpub.com/shelf/buy :)
Thanks very much to all our authors who have supported us over the years!