If I could pick three books that every serious Christian would read and own for repeated reference, they would be:

1. N.T. Wright’s Surprised by Hope. Can’t we please leave behind all the Left Behind garbage?  That isn’t what Paul, Peter, John, Jesus, etc., meant.  What is the Christian’s true hope?  How do we live it out faithfully as God’s image-bearers here?
2. John Walton’s Life Application commentary on Genesis.  It’s not that I agree with all of Walton’s conclusions–I do not, I disagree with him multiple times.  But this is a faithful, well-written, accessible treatment of this most foundational book of the Bible that attempts to treat Genesis as ancient near eastern literature.  Walton does a great job of discussing the worldview issues around this most-misunderstood part of scripture.
3. Brennan Manning’s Ragamuffin Gospel.  “Nothing in my hands I bring; simply to thy cross I cling.”