Any text editor works. For programmers, we recommend writing in Emacs, vi or TextMate. For non-programmers, we recommend iA Writer for Mac and iPad. If all else fails, you can also use TextEdit on Mac or Notepad, Wordpad or Notepad++ on Windows.
Here is a thread on this subject from our Authors Forum: https://community.leanpub.com/t/offline-editor-recommendations-experience/1962. Please feel free to add your own preference to the thread!