Book about Christ
Centrally, the Old Testament is a book about Christ, and more specifically, about his sufferings and the glories that will follow—that is, it is a book about the promise of a coming Messiah through whose sufferings God will establish his glorious, eternal kingdom.
Iain Duguid, "Old Testament Hermeneutics ," in Peter Lillback, ed., Seeing Christ in All of Scripture: Hermeneutics at Westminster Theological Seminary (Philadelphia, PA: Westminster Seminary Press, 2016), 17