You can add epub files to iBooks by right-clicking the file in Finder and selecting "open with" then clicking iBooks.
iCloud can also be used to redownload books or unhide them.
Additionally, you can tap the attachment icon to download the file to your device, and then tap and hold on the file to open it in iBooks.
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Apple Books is a great way to read books and customize your reading experience. It allows users to buy and download books, organize their library, and sort their books. If a book is not in your library, users can turn on iCloud and redownload it, or unhide it.
Read books and more with Apple Books - Apple Support