Since you ask for a
single book, I would say that one's most important task in the Christian life is to develop an ever-deepening relationship with the Lord. All apologetics, charitable work, ministry of any kind, and even intellectual development must arise from a profound interiority if they are to be fruitful. Toward that purpose, I can think of nothing better, or more concise, among the classic texts than Thomas à Kempis'
The Imitation of Christ.
Hope this helps with your lectio divina!