If you need to transmit a mass of iBooks or partial selected data from iPhone to computer, I recommend you to use an unexceptionable third party program - iOS Transfer, which enables you backup and transfer iPhone, iPad and iPod Touch books (you purchased via iBook Store) to your computer.
Here is the direction on transferring books from iPhone to computer. In the beginning, you need to make a preparation - downloading and installing iBooks Transfer.

Easy Guide: How to Transfer iBooks from iPhone to Computer?
• Step 1: At beginning, download and install iOS Transfer
Open the iOS Transfer, after installing the program, please connect your iPhone to your computer and launch the program. There are the reload, add and setting on the topmargin.

• Step 2: Choose books of iPhone
After installing iOS Transfer, you need to click the "iBooks" button on the left column, you can see some informations of your books like name, author, size and so on. Right-click on the left of books' name in that small blockage

• Step 3: Transfer and iPhone iBooks to computer
Click the "To PC" button after selecting books you like, then find a folder where you like to save or you get used to using save files, just click the "OK" button after choosing, you will see your books in your computer after few minutes.

Above are the detailed steps of how to transfer iPhone books to computer with iOS Transfer. The program is quite powerful and suited to mass data transmit and backup. If you connect your iPhone to your computer, it can help you to transfer iBooks with ease.
iOS Transfer (Mac) support all types of iPhone, iPad & iPod Touch. Apart from iBooks, the program can enable you to transfer music, video, photos, contacts, sms, podcasts, tv shows and so on.
Download iOS Transfer, which can help you manage all your iPhone iBooks to computer on Windows/Mac now.

Related Guides & Articles
• How to Transfer Files from iPhone to Mac
• How to Transfer Music to iPhone Using iTunes
• How to Transfer Video from iPhone to Computer

