I have a friend who strongly asserts they are a believer. This despite not reading the Bible, not going to church, fornicating, etc. This person is a voracious reader, so I was hoping I could could get them a good book about what it means to be a Christian (at a beginner's level) that God might graciously use to open his person's eyes to their true state. Any recommendations for spiritually challenging, entry-level books about what it means to be a Christian?