The first book I would suggest he begin reading is the Bible, starting with one of the four Gospels of the New Testament. (I often recommend the Gospel of John for someone who doesn't know much about Jesus, because it was specifically written for people like them.)
The reason is because through its pages he will discover Jesus - who He was, what He came to do, and how we should respond to Him. Is anything more important for him - and us - to understand? People like your daughter's boyfriend often have all sorts of ideas about Jesus that aren't actually true - ideas they've picked up from conversations, movies or other sources.
But when people discover who Jesus really is and what He did for us through His death and resurrection, those distorted ideas begin to vanish and they want to give their lives to Christ. As the Bible says, "These are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name" (John 20:31).
Your Christian bookstore can suggest other titles, of course, if your daughter's boyfriend still has questions. But do all you can to point him to Jesus. Realize too that by your example, you and your husband are like "living books" to him. Does he see Christ in you? The strongest argument for Christ is the reality of a changed life.