Tool Tips
So you don't want to give money to Microsoft, and you want a good word processor. OK, that's easy. LibreOffice.
Now you want to produce yuour own EPUB format eBooks. Well, that's OK, LibreOffice has an EPUB export. Simple, right? And the EPUBs it generates work. But ... they have some issues. Which you want to fix.
So you go into the EPUB.
DEAR MOTHER OF HASTUR RIDING ON A FLAMING VELOCIRAPTOR, WHAT IN THE NAME OF EVERYTHING CURSED AND UNHOLY AM I LOOKING AT?
LibreOffice generates atrocious EPUBs.
Or, more accurately, LibreOffice hands off the job to libepubgen. Which is a piece of alpha-quality v0.1.1 abandonware that hasn't seen a single code commit in three years. What it generates technically works, but dear gods, it is a Superfund site of bad markup.
"Please tell me there's a solution to this!"
There is a solution to this.
The solution is to install a tool called pandoc. That's not the WHOLE solution, because pandoc's .ODT reader has some serious bugs. BUT: If you save a copy of your LibreOffice .ODT document in .DOCX format, you can then use pandoc to convert that DOCX file to EPUB (actually, you should specify -t epub3), and you will end up with an EPUB 3.0 eBook that it is meaningfully humanly possible to edit and format.
And then, basically all you should need to do is go in and fix up the CSS to make it look the way you want it to look.