If you value your ebook collection and use iBooks on your reader, I suggest you don't upgrade to Apple OS X Mavericks.
Luckily, I had a Time Machine backup and was able to restore my system to the old OS.
The new iBooks for Mac is the default ebook app in Mavericks. Meaning you can no longer use iTunes to manage your books.
Upon opening the app (no, there's no warning to backup your files, etc.) it moves all of your files in iTunes "books" folder into iBooks AND RENAMES THE FILES. All of your books will have alphanumeric gibberish as their filenames. The author folders are gone. The book files are stored deep into the operating system.
There's more issues, sync to iPad/iPhone not working, PDFs missing, metadata gone. See the apple discussion forum for more info.
https://discussions.apple.com/commun...ibooks_for_mac
(Can a Moderator move this to the Apple Devices section? Oops.)