Zipped Website Container/Organizer for the eReader
If the book you are writing were a web site, it would have parts like:
So ..., "yourbook.epub" could be opened in an Apple iPad, for instance, as a book container with all those parts inside (and maybe more).
Word Processors that Export EPUB
Children's eBook Help
How to articles ...
Pages to ePub Direct Export
Every quality word processor should have an 'Export' or 'Save As' feature for the .epub file extension and structure.
Apple's iWork Pages software is ideally suited to this export feature evolution because it already works well with the XML code that .epub uses.
MS-Word has an excuse for being slow to adopt the Export as EPUB feature because of their .docx court case setback.
More in the future ...