I was a fan of Cardinal Ratzinger before I was a fan of Pope Benedict. He is an absolutely brilliant thinker, maybe even more so than PJP II, who I think had three PhDs and wrote some of the best encyclicals in history.
Most of his books are published by Ignatius Press. I’d recommend anyone who wants a first impression to look at his early book, The Meaning of Christian Brotherhood, which has earned praise from Evangelical theologians.