Robert B. Parker's "GOD Save the Child" with T.S. Eliot's "Murder in the CATHEDRAL".
I urge you to be very creative. Animals, elements of nature, colors, sport, religion, whatever jumps out at you. If you're really on the ball, perhaps the release point could factor into the theme, too.
Books should be left together, perhaps with a rubber band or ribbon around them, to signify a double-release.