What is the main point of the book of 1 Peter?
This a a great question. I think if you look at the scholarly literature, then there is good consensus that the main point of the book of 1 Peter is the call to suffer well for one's faith in God.
Here are a few interesting facts. The book uses the name of God more than any other letter in the New Testament, which is significant is such a short book. And the Greek word for suffering (pascho) is used in this letter more than the all the other letters combined. This should give you insight into what Peter's main point really is.
In light of this, 1 Peter can be seen as a manual on how to suffer well for one's faith. From a Christian point of view, this is an essential lesson, because the call to faith is a call to imitate Christ, who died and rose again.
Here are an extended quote that underline the theme of suffering from 1 Peter 2:21-23:
21 To this you were called, because Christ suffered for you, leaving you an example, that you should follow in his steps.
22 “He committed no sin, and no deceit was found in his mouth.”
23 When they hurled their insults at him, he did not retaliate; when he suffered, he made no threats. Instead, he entrusted himself to him who judges justly.
It is a short book, it might be worth reading. It should take you 20 minutes.