I had purchased an iBook years ago from the store and it is no longer available now. I am unable to dowload it to any device, including my computer, off of the cloud. Also, what is interesting is that I have a copy of it downloaded onto my iPad (saved in iTunes) but it WILL NOT READ on my iBooks for Mac because of it being off the store. I was a little concerned so I wanted to back up my iBooks to pdf's should this ever happen again. I came to find out it was challenging to find good quality guidance on the issue. So, I figured it out.
As of 11/6/13 with the new OSX Mavericks installed along with the latest iTunes 11.1.3 and new iBooks app for mac here is a quick way to do it.
1. Do a spotlight search (CMD + SPACE) to find the iBooks folder. (it is in your documents)
2. Click on the folder and open it, inside you fill find another folder called Books.
3. From here you may have to do some guesswork to find out which #######.epub file is the book you are looking for. I suggest trying to open them individually and going on from there.
4. Once you isolate the file, make a copy (option hold, click and drag) to the desktop.
5. Go to this website http://www.convertfiles.com/convert/ebook/EPUB-to-PDF.html
6. "Browse" for your file, select the ####.epub on your desktop. Make sure .epub is the "input format" and .pdf is the "output format"
7. Click convert.
8. Click the external link to download your converted file.
Happy reading, where you want to, with backup!