Of the 30 books on the front page of your bestselling list, the closest things to an 'academic book' I see are all texts designed to accompany Coursera courses.
Those aren't academic books, those are introductory text books.
Those aren't academic books, those are introductory text books.
Leanpub has also been used to publish work based on dissertations (which is as academic as it gets), and those works obviously have far fewer sales.