"The Purpose-driven Life" (Rick Warren). I told my boss, "The library's selection of Christian books is filled with heretics and apostates." Given what I've heard of "The Purpose-driven Church", I was with great trepidation that I picked this book. That said, I found PDL to be ok overall. Of course it is very much, "God loves you and has a wonderful plan for your life" and "No one is truly happy without Jesus". But it has some refreshing moments. The first sentence is, "It's not all about you."
I would not recommend PDL for non-Christians. But for those who are already Christian, it could serve as encouragement and motivation to become more active.