iTunes 
Add iBooks and audiobooks to iPod, iPhone, or iPad
You can download iBooks to your computer and use iTunes to sync them to your iPod touch, iPhone, or iPad. You can also use iTunes to sync audiobooks to your iPod or iOS device.
Sync iBooks or audiobooks to your device
Connect your device to your computer and select your device in iTunes.
Click the Books button.
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Do either or both of the following:
Sync iBooks: Select Sync Books, then click “All books,” or click “Selected books” and select the iBooks you want to add.
Sync audiobooks: Select Sync Audiobooks, then click “All audiobooks,” or click “Selected audiobooks” and select the audiobooks you want to add.
Click Apply.