An EPUB book has multiple chapters. What method to split it into multiple files, each of them consist one chapter, effectively?
If each chapter is formatted as an h1-h6 tag or marked with a unique style, you could use Sigil to insert split markers. For example, if all your chapter headings are formatted as
<h1> tags, you'd use:
<hr class="sigil_split_marker" /><h1
Then select Edit > Split at markers.