2 Corinthians 12:9 says, My grace is sufficient for you, for power is perfected in weakness.
These books were divinely inspired by the Holy Spirit. His intention for this addition is to bring spiritual maturity into the born again believers life. It is also intended to give an understanding to the non-believer that God is for you and not against you.
The true definition of grace is, "The divine inspirations of God." By His inspirations into our lives, we become the holy people of God, and are able to do all that the Lord has called us to do.
Throughout the almost 30 years that I have been a born again believer, I have experienced the Lord, and Him doing some amazing things. They were all done just by His inspirations into my life, or through someone else's, and all it took was a simple act of faith in response to Him.
I believe He wants to do something amazing in, through, and for everyone that will accept His Son Jesus as their Lord and Savior.
What a mighty God we have, and many of us serve.