OK, so since no one chimed in on this its down to me to just fill in the blank. So, if I could only give an unbeliever one book what would it be? I would also have to go with Piper's Desiring God (as Reformed Christian UK did) and I'd follow that up a short time later with his Don't Waste Your Life. Both are fine books for a believer at any stage in their walk.
Piper has such a great balance to his preaching and teaching, he just can't be beat.