If your iTunes books are not purchased from Apple but are DRM-free EPUBs from elsewhere, you can add the books from the originals.
If you lost the originals, then you will have to hunt down where iTunes keeps the books, so you can point calibre at those files.
You are probably still screwed, because the latest version of Mac OSX has iBooks, which you appear to be using, which is bad because Apple decided to rip apart all iTunes books and stick them in iBooks storage as obfuscated fragments.
You cannot piece them back together into something other programs will understand -- or at least, no one to my knowledge has ever done it.
If the books were purchased from iTunes, then you have just learned why people Never Ever Ever buy ebooks from Apple.
You cannot retrieve them, sorry.
What is the moral of this story? Well, I am the type of person who believes all Macs should be set on fire
but one thing
everyone agrees on is that when it comes to ebooks, Apple is just plain evil.
