Last winter my church used Tim Keller's The Prodigal God as the basis for an "all-church study" — 6-week preaching series, Sunday school class, and small group discussions. It went well and we are considering doing it again. It's not so easy, however, to find a book that works as well as Keller's Prodigal God — short & accessible, yet profound & practical. Other books that I've considered are The Call by Os Guinness and Engaging God's World by Plantinga. Any other suggestions?