I asked a similar question a few years ago, 'Why is Philemon in the Bible?'
It is a book that speaks of the need to forgive. Onesimus was the slave of Philemon. For whatever reason Onesimus was separated from Philemon (He might have ran away.)
While Onesimus was away, he met Paul and became a believer. Paul could have really used Onesimus in his ministry, but knew he must do the right thing. So, Paul wrote the letter to Philemon requesting that he accept him back as slave and most importantly as a brother.
The key verses are:
"15Perhaps the reason he was separated from you for a little while was that you might have him back for good— 16no longer as a slave, but better than a slave, as a dear brother. He is very dear to me but even dearer to you, both as a man and as a brother in the Lord."
The themes are the need to do what is right, the power of forgiveness and the remembrance that God has forgiven us more than we can ever pay back.