It's easy to un-archive a course!
First, go to the Courses section of your Leanpub library here:
https://leanpub.com/user_dashboard/courses
At the top of the page, you will see a dropdown menu. By default, "Current" will be selected:
Switch the dropdown menu to "Archived"
Next, select the course and click the button to un-archive it. The course should now be back in your library here: