I'm writing with manually line-wrapped paragraphs, and Leanpub is adding single line breaks for my line wrapping. How can I turn that off?

For line-wrapped Markdown books, we recommend Leanpub Flavoured Markdown mode (https://leanpub.com/lfm/read) not Markua mode (https://leanpub.com/markua/read).
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Leanpub's Markua support currently doesn't support treating single newlines as spaces. It will eventually, but not for a number of months.
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Learn more, and how to switch writing modes, here:
​http://help.leanpub.com/en/articles/2941344-leanpub-flavoured-markdown-vs-markua-for-writing-in-plain-text