ONE book! That’s tough.
I’m going to say Tim Keller, Prodigal God: Recovering the Heart of the Christian Faith. One of my all-time favorites. But more than anything, if you haven’t read Keller, it introduces you to his writing. But it’s a favorite because it addresses the centrality of the gospel in the most unique and refreshing way.
If you’ve read that one, then a more challenging but incredible book is Keller, The Reason for God: Belief in the Age of Skepticism. You’ll not only be encouraged, but you’ll draw on it for sermons for years.
And if you’ve read those two, my suggestion is a good overview of the Bible from a biblical theology standpoint. Two very accessible but good books for that are Tim Chester, From Creation to New Creation: Making Sense of the Whole Bible Story, or Vaughan Roberts, God's Big Picture: Tracing the storyline of the Bible.
I didn't have a lot of time to think about it, but what would you recommend?