OT: There are two: Tobit. I must note that the Vulgate-based Tobit (Douay-Rheims, Knox) is even more endearing than the later versions, which begin in 1st person then suddenly and inexplicably switch to 3rd person. Tobit speaks to me so deeply that it normally brings tears. Tobit sheds light on the “brothers of Jesus” pseudo-controversy by pointing out that a man’s wife from the same tribe was called his sister. Anyone from the same tribe, which is how ancient Hebrews associated, was a brother.
Wisdom is precisely that. It has perhaps the clearest prophecy of Christ in the entire OT (Chap. 2), specifying His nature, thoughts, teaching, torture and criminal’s death. References that apply to Christ may be seen in chapters 2, 3, 4 and 5. It was written about 50 years before Christ. So much for that alleged “inter-testamental period” spoken of by those who reject the Deuterocanon.
NT: 1 & 2 Peter, as they are doctrinally dense and clearly show the Holy Spirit speaking through the first Holy Father.