You have a book with .epub or .mobi format in your MacBook or PC.
You want to put it into your iPhone or iPAD so that you can read it on the go.
1. Put the books into iCloud Drive.
2. Install Kindle or Apple Books on your iPhone or iPAD.
3. Login your Kindle account.
4. Open the Files app to go to iCloud Drive.
5. Open the book.
6. Tap the Share icon in the lower left hand corner.
7. Select Kindle or Open in iBooks.