[bravosx]

@slowsmile...
I made the LibreOffice convert the file from the link that was sent in by Doitsu in post #16 saving as .odt.
It was the output file to further transformations using Save As>Document HTML (Writer)(.html) setting in the Tools tab>Options>Load/Save>HTML Compatibility additional character sets:
Western European (Windows-1252/WinLatin1)
Central European (Windows-1250/WinLatin2)
Unicode UTF-8
In all ways of Polish letters in the book now show up correctly. Only problem is the file contents.xhtml (TOC), where Polish letters disappear completely.

@Doitsu
I have prepared all the files you requested, I can send but I do not know how to do it.

Regards bravosx