Where are books stored after iBooks migration on Mavericks?
They are now buried inside the user Library folder.

The full path is:
~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books

The Books.plist there contains itemID values that connect the book titles with their *.epub or *.ibooks names [iBooks 1.2, OS 10.10.5].