How to move my iBooks library from one mac to another?
I couldn't figure out where exactly iBooks stores imported books but I found out something else that might help you.

Select all books you want to backup and then drag them out of iBooks onto a Finder window and they sould be copied over.

I am not sure but it seems unlikely, that your reading progress and/or bookmarks are being copied as well. If you don't mind that, than this would do the trick. 

Otherwise someone else might be able to locate the folder where iBooks stores the books. I found this answer to that question but on my Mac it didn't seem to be correct: Link