Like the Book of James (of Letter of James), the Letters from Peter are open letters written to Christians spread far and wide, to give them guidance in maintaining their faith and how to understand and endure suffering for their beliefs. The letters were written after the apostles (first followers of Jesus) were dead and it started becoming difficult to keep the truth in tact. With false prophets out there trying to corrupt Jesus's message, it was important for Peter and James and the other new disciples to spread what they believed was the truth.
It's in these letters that Peter talks about salvation as like being "born again," to renew one's life after hearing and accepting the teachings of Jesus.
"Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently: Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever." -- 1 Peter 1:22-23
Suffering is something Peter talks a lot about as well. To live as a Christian is to set oneself and others to a high standard, being patient, respectful, humble, even if it means suffering and not getting one's way all the time. Christians from the beginning were also persecuted for their beliefs and faith in Christ, and Peter reminds them that to suffer is to follow in Jesus's footsteps and be honoured to share in his work, by serving God.
"For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow in his steps." - 1 Peter 2:21
Peter's second letter focuses on how being a Christian is not a one-time act to be checked off a list and forgotten, but rather a life-long process. Peter urges everyone to be faithful, good, wise, have self-control, perseverence, kindness, and love. (2 Peter 1:5-7) He reminds the new Christians that God will sustain them in their lives if they trust Him, like He did with Noah, whom God saved from drowning because Noah was a good guy.
Finally, Peter ends with the reminder that Jesus would come again, and that while many people were becoming cynical because he hadn't returned yet, despite having been resurrected for many years, God's time is not like ours. For God, Peter says, a thousand years might be like a single day. Peter says we need to be patient, be ready, and in the meantime, "grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ." -- 2 Peter 3:18
**NEXT: I, II & III John