Moving iBooks library to an external drive
You can copy, then symlink, the iBooks folder to an external drive.

The iBooks library is stored in the following location:

~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books


Quit iBooks, then move this1 folder to an external drive. Then, symlink this folder:

ln -s /path/to/iBooks-library ~/Library/Containers/com.apple.BKAgentService


1 In the example above, I moved the entire BKAgentService container, but you can just move the books themselves and leave the rest on the local disk — may improve performance.