This was a no-brainer for me, it's Shane Claiborne's The Irresistible Revolution: Living as an Ordinary Radical which made me change my way of thinking about many things. It was funny and easy to read, was informational - I had to break out my highlighter, and it has changed me.
His message of simplicity is a great one in this age of materialism. Although doing everything his way would not work for us and I don't agree with all that he says, many of his ideas have hit home. It certainly does challenge you. If a book can do that, I believe it was a good choice!