Also, you can save as HTML in LibreOffice, but writer2Epub will give you an actual epub which will be cleaner than the HTML save. But, like with Toxaris' addon, I believe it then requires you to define what each style should look like. (or go with the defaults.) Both addons focus on cleanly exporting the actual words of the book, and the styles you applied to sections, while tossing the junk that invariably creeps in when using a pure WYSIWYG editor.
Experiment a little and find what works best for you!
