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.

https://www.quora.com/How-do-you-write-a-reference-to-a-self-published-book-according-to-the-Harvard-referencing-system

http://classroom.synonym.com/cite-memoir-2968.html

https://answers.yahoo.com/question/index?qid=20150114080404AAv4g8m

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