This book is a plea that God himself, as revealed most clearly and fully in Jesus's death and resurrection, be seen and enjoyed as the final and greatest gift of the gospel.
The gospel of Jesus and his many precious blessings are not ultimately what makes the good news good, but means of seeing and savoring the Savior himself. Forgiveness is good because it opens the way to enjoying God himself. Justification is good because it wins access to the presence and pleasure of God himself. Eternal life is good because it becomes the everlasting enjoyment of Jesus.
All God's good gifts are loving to the degree that they lead us to God himself. This is the love of God: doing everything necessary, most painfully in the death of his Son, to enthrall us with what is most deeply and durably satisfying--namely, himself.