Sigil is one of the most powerful ebook editor in circulation, it’s totally free and opesource, but it cann be tricky if you don’t know how to work on the source code of an ebook.
What I offer you:
Checking over html formatting and error correction.
Design improvement through CSS (images, drop caps, indentation, chapter decorations).
Adding interactivity (internal links, TOC, index, notes), audio and video files (for ePub3).
Metadata optimization and semantic tagging for better indexing.