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You bought an e-book from somewhere that isn't Amazon, or a friend sent you a digital book. It's an EPUB — the open format that every major e-reader supports. Apple Books, Kobo, Google Play Books, and Calibre all handle EPUB natively. Amazon's Kindle held out for years with its own proprietary formats, but newer Kindles finally support EPUB too.
The format itself is quietly brilliant: an EPUB is a ZIP archive containing HTML, CSS, and images — essentially a tiny website packaged as a book. Text reflows to fit any screen. Fonts resize. The reading experience adapts from phone to tablet to e-ink display. You can crack one open with any ZIP tool and read the HTML inside with a browser if you're curious (or desperate).
Apple Books (macOS/iOS) and Calibre (free, all platforms) are the best readers. Calibre is also the Swiss Army knife of e-book management — it converts between EPUB, MOBI, PDF, and a dozen other formats. For publishers and indie authors, EPUB is the format that keeps books free from any single company's walled garden.