There is, sort of, but iBooks makes this difficult to all but the most stout-hearted.

If you visit `~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books/` on your Mac, you'll find extracted versions of your `.epubs`, if you've downloaded them into your iBooks Library. The problem is, that in order to be valid ePubs they'd need to be archived again (as a .zip renamed to .epub, which is all ePubs are).

Also, iBooks uses DRM on paid(?) books downloaded from the store, making this more complicated even then.

So, yeah. I've plans for a desktop client that might make importing from iBooks possible/easier, but I'm a little afraid to go barking up that tree.