"Christ Our Joy" by Msgr. Joseph Murphy is the first Catholic book request that I successfully made to our local library! Periodically, I receive suggestions from the email list called "Pope St. Nicholas V" at Yahoo Groups. So I immediately checked this out, but found it more academic than I expected (i.e. it took me longer to read it). It is not written by Pope Benedict XVI, but takes quotations from his many writings all his life. He uses 5-6 chapters to show how true joy is rooted in the Trinity. Here are my two favorite bits:
"It must be clear that pleasure is not everything, that Christianity gives us joy just as love gives us joy. But love is also a renunciation of self."
"Joy grows and continues to mature in suffering in communion with the Cross of Christ. It is here alone that the true joy of faith is born."