I am always interested to see how other interpreters sum up a book of Scripture. Here is one for 1 Peter.
“It was the common practice of the early Church commentaries on the Bible to identify the aim of a given biblical book right from the start. What then is the aim of 1 Peter? Peter writes to the churches in five regions (see 1:1) to prepare them for suffering in imitation of Christ. As members of God’s household, they need to know their new identity in Christ, learn how to relate to others both within and outside the Church, and be ready to undergo affliction for their faith. In fact, the characteristic feature of the letter is the sharp contrast between the sober call to suffer in imitation of Christ and the ‘indescribable joy’ (1:8) that is ours because of our new standing in Christ. Like a sympathy that moves back and forth between major and minor keys, 1 Peter oscillates between the expression of profound joy on the one hand and the call to endure trials on the other. Peter’s aim is to show that the Christian life, characterized by a living hope and deep joy, will also be marked by suffering for the sake of Christ.”
Daniel Keating, First and Second Peter, Jude, Catholic Commentary on Sacred Scripture (Grand Rapids: Baker, 2011), 17–8.