The King James Bible The Book of Common Prayer of 1662 Those are the only must have Christian books. In the useful category, I can recommend anything by C.S. Lewis, including the Screwtape Letters.
I agree, that's about it for 'must-have' books. But there are plenty of other useful books. Sometimes which ones will depend on personal interests. A few books I personally found valuable: "Why I Believe" by D. James Kennedy "The Miracle of the Scarlet Thread" by Booker "Christian Theology" by McGrath "Smith Wigglesworth: Apostle of Faith" by Frodsham "Was Christ God?" by Zodhiates