Amazing Grace by Eric Metaxas. I know, I know, definitely read Bonhoeffer, too. But I had read this first and loved the author's warm tone and very readable account of William Wilberforce. And yes, I mainly watched the movie because Ioan Gruffudd was in it, but I was so glad I read this as well. Wilberforce was truly a force to be reckoned with, and ahead of his time as a champion of social justice.