I love reading well written biographies of figures from church history. I recently finished Augustine of Hippo: A Biography (WTS Books | Amazon) by Peter Brown and I think it's safe to say it's now one of my favorites.
At nearly 600 pages it isn't a quick read, but Brown knows how to keep a reader's attention. If you'd rather start with something smaller on Augustine I recommend John Piper's The Legacy of Sovereign Joy (free PDF or book).
Ryan Wentzel