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calibre
Open and organize EPUB books on your computer
Desktop appFree ebook reader and library · bring your own books
Open calibre Why this one
Keep downloaded books organized by author and title and read them in one desktop library. Useful when a folder of EPUB files is harder to navigate than the books themselves.
Try this first
Add a public-domain EPUB or a book you are entitled to use. Check its title and author, open it in the viewer and keep a backup of the original file.