After hours of research, I finally find the hidden path where the iBooks EPUB files are saved. The path is "/Users/~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks".
To make the Library folder visible, please open the "Terminal" on Mac, then input "chflags nohidden /Users/~(your user name)/Library".
Marsattacks's tip to show the Library folder in an easier way: Just select user in finder sidebar, click 'command j' to show options, and select 'show library'. - Thanks, MarsAnd that is not the end of the job. By entering the deep iBooks folder, you will find the books are not saved in .EPUB extension, but a folder with the name ended with .EPUB.
Don't worry, those folders ended with .EPUB are actually your EPUB books, to get a complete .EPUB file, just zip the folder and make the extension name as ".epub", then you can get an complete .EPUB file from iBooks for Mac app in Mac OS X 10.8 Mavericks.