The iPhone and iPad are great devices for reading ebooks. You can use the iBooks app to read books in the ePub format, including iBooks purchased from Apple. But you may have your own ePubs that you've purchased from other vendors, or even public domain books that you've downloaded. If you don't want to use the combination of the iBooks app and iTunes (on the Mac) or iTunes alone (on a Windows PC), iMazing can transfer your iBooks and ePubs to your iOS device. Here's how.
Here's how to transfer iBooks and ePub books to your iPhone or iPad:
Download and install iMazing on your Mac or PC computer
Download and install iMazing. Launch the app, and connect your device to your Mac or PC.
Select your device in the sidebar, then select iBooks. iMazing loads your device's ebooks.
iMazing displays a standard Open dialog prompting you to select a folder on your Mac or PC.
Choose a single ebook, or, if you want to import a folder of ebooks, select that folder. If you want to import multiple folders of ebooks, you'll come back to this step after each folder has been transferred.
iMazing's Operations window shows the progress of the transfer, then, after the ebooks have been copied, reloads its window showing all your ebooks. You can then quit iMazing and read the ebooks on your iPhone.