Here's an article about converting LibreOffice to EPUB format:
ClickHere
The article provides a download of a fairly complex input document in ODT format to test conversions to EPUB.
I ran the document through calibre conversion (ODT -> EPUB) with good results. Pertaining to calibre, the article said:
"Good output, competitive with eLAIX".
Are your books in ODT or ODF format? Does LibreOffice let you open an ODF and "save as" ODT? If so, using LibreOffice for ODF -> ODT and then calibre for ODT -> EPUB might yield the most accurate conversion without a lot of manual tweaking.