Markua and Leanpub Flavoured Markdown are the two ways of writing in plain text on Leanpub. Both are dialects of Markdown. Markua is newer and better than Leanpub Flavoured Markdown, but there are still some advanced features in it that aren't finished yet. However, even though our Markua support isn't complete, it already has more features than Leanpub Flavoured Markdown.
To write in Markua or in Leanpub Flavoured Markdown, you need to be in Dropbox or GitHub writing mode, or you need to be writing in our in-browser text editor.
If you have an existing Leanpub book and you want to switch writing modes, you go here:
Author > Books > (your book) > Settings > Writing Mode
Once you've chosen an appropriate writing mode, you then can switch between the Markua and Leanpub Flavoured Markdown dialects of Markdown here:
Author > Books > (your book) > Settings > Markdown Dialect