Start iTunes, go to preferences, and see if the Book option is selected in preferences.
After connecting your iPhone, iPad or iPod touch to your computer with a data cable, select and view the book categories on the left side of iTunes. If there is no such classification, restart iTunes and repeat the steps checked above.
Drag the PDF or ePub e-book you want to import to this blank area, and iTunes will automatically load the e-book. When finished, you can see the following interface.
Select the device connected on the left (iPhone, iPad or iPod touch), select "Books" on the right of iTunes, check "Synchronize Books", continue to select the e-books we want to synchronize, or select all of them.
Finally, click the application in the lower right corner of iTunes and wait for iTunes synchronization to complete.
Go back to iPhone, open iBook, find and read the e-book you just imported.