What is the Gospel? by Mandy Groce and published by Christian Focus Publications.
God has some good news for you and it's called the Gospel. "What's the Gospel?" you say. It's the good news that even though we sin and have disobeyed God, we can have life and joy as part of God's kingdom. You see I don't deserve God's goodness, but he reached down and rescued me even when my sin had made me his enemy. This book explains what the Gospel is - not just parts of it.