.epub — Electronic Publication
Document application/epub+zip
What a .epub file is
The open ebook standard — essentially a ZIP of HTML and CSS, so text reflows to fit any screen size.
How to open it
Apple Books, Calibre, Google Play Books, most e-readers except Kindle.
Worth knowing
Kindle uses its own formats. Calibre converts EPUB to them, or you can email the EPUB to your Kindle address.