Lives of the Popes by Richard P. McBrien

A pretty good read once you filter out the author's liberal bias. The author is a liberal theologan who seems to believe that religious doctrine should be made by consensus, we sure wouldn't have that pesky commandment on adultery if that was the case. He also applies 21st century liberal bias to the actions of popes almost two thousand years ago. Still, once you read around the bias, pretty well researched and written.