I'm actually not sure how different citation systems deal with things like books bought through Leanpub.
Based on a bit of research, here's one example of how it can be done:
Epp, Leonard. The Rhetoric of Obscurity. n.p.: Author, 2016.
The "n.p." means "no place", and stands in where you would normally put the publisher's place name, e.g. "London". "Author" is what I gather one should put in place of a publishing company's name, when a work is self-published (Leanpub is a platform for publishing, not a publisher).
We're familiar with the importance of getting this right by following explicit guidelines for a known reference style in a given discipline, which we would recommend you do.
To get a better answer than mine, you could try going to a university library and asking a librarian about the situation. In my experience librarians like to be asked about things like this.
PS Here are some resources I found useful. Curious how little their is about this to be found.