How to reset iBooks app in OS X 10.10
In Terminal.app (found in the /Applications/Utilities/ folder), run this command and arguments:

/usr/bin/defaults delete ~/Library/Preferences/com.apple.iBooks


This will wipe the plist file, but may not trash it.

Now when you open iBooks, the settings will be back to the default.

This same command will work with other apps using the appropriate arguments.