Mercy: Essence of the Gospel and the Key to Christian Life
This is the book pope Francis read around the time of his election and recommended during his first Sunday Angelus: "this book has done me so much good", repeated three times. I haven't read it, but it should be pretty essential for anyone really wanting to understand his thinking.
Walter Kasper was the oldest of the Cardinal electors at the conclave. He is president emeritus of the Pontifical Council for Promoting Christian Unity, and has become something of an unofficial papal theologian for this pontificate.
More of his books can be found in the Walter Kasper Collection (2 vols.) and the T&T Clark Studies in Systematic Theology (6 vols.).
