We list our books in one currency for customers all over the world, specifically in US dollars.
In terms of why we use USD, the short answer is that it is best default choice of currency for most people.
(Ironically, we're based in Canada, in Victoria, BC.)
Are you looking for great deals on Leanpub ebooks, ebook bundles, and courses? Sign up for our Weekly and Monthly newsletter sales here!
Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book