Where are the sample books stored in the iBooks for Mac OS X? I'm doing an accessibility stuff and I need to access the source file (.epub file) of the sample books in iBooks for Mac OS X. I did look for one at ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks but that only stored bought books. Does anyone know?

(My OS is the newest version of Yosemite.)
Here you can find the iBooks source files:

Go to Library > Containers > com.apple.BKAgentService > Data > Documents > iBooks

Or simply type 'iBooks' in the searcher of Finder and click on the folder named iBooks.