If not, open "Edit"-"Preferences" in the upper left corner, put a tick in front of the "Book" option below, and then click "OK".

Click the "Book" tab to find the epub book just now, click and drag it to the iTunes box on the right, and then release the mouse until "Copy" appears. We can see that this book already exists on the right side of iTunes.

Link the device of the mobile phone to the computer, select "Books" on the right side of iTunes, check "Synchronize Books", and select the e-books to be synchronized.

Click Apply in the lower right corner of iTunes and wait for iTunes to sync.

Open iBook, an e-book software on iPhone, and you can see the e-book just synchronized.