Work through this in order
- Confirm the file is a ZIP-style EPUB container
- Find the mimetype and META-INF files
- Inspect the list of OEBPS or EPUB content parts
- Use a dedicated e-book reader to view styled chapters
- Do not rename broken archives and assume they are valid books
Use the matching tool
Inspect a local copy with EPUB File Inspector. After the check, save or repair files only using trusted software that supports the format.