The Popes: Histories and Secrets. I bought this book quite a long time ago as a reference book. It is no longer in print, but pretty easy to find in used book stores. The Popes is a chronological description of the personalities and histories of the 264 popes in Catholic Church history previous to Pope Benedict XVI.
If you're looking for quick information about papal history, Wikipedia has many articles on the office in general and the specific popes who have held the office. For instance, you can read about Gregory VII, the most "recent" case of a pope abdicating the papal office (that was 1415!) prior to Benedict's retirement. You'll find a complete Wikipedia list of Roman Catholic Popes HERE.
To get a primer into the thinking and writing of Benedict XVI, you might want to check out The Essential Pope Benedict XVI: His Central Writings and Speeches. The sections of the book include: The Church, The Liturgy, Theology, Scripture, The Priesthood, and Christian Morality. In his early years, Joseph Ratzinger was considered a liberal theologian, but after 1968 his views on culture and theology became conservative. He is one of the more prolific popes in church history. His writings typically defend traditional Catholic doctrine and values while countering the Western secularism he believed to be so destructive in the 20th and 21st centuries.
If you're interested in learning how to become the next pontiff, you'll find this video to be helpful.