In my opinion, this book is about how important it is for people to be able to think for themselves and to feel emotions. So I would try to make my summary for the back of the book convey that idea.
I would say something like:
All his life, Jonas had lived a very good life. He had never really been unhappy in his whole life. But one evening, he asked his parents if they loved him. They could not answer.
In this story, we find out why Jonas's parents don't really love him. We find out why he suddenly asks if they do. And we find out what impact his discovery of love and emotions has on his community.
Jonas was a young boy who could not wait until he turned twelve. He had grown up in a place where he had lived a happy and common life. He was excited that he would soon be given an assignment that would help him to know what he would be doing in his future.
In the community that Jonas lived in everyone dressed the same, ate the same, lived the same. Different people had different jobs. His parental unit father was a nurturer in a nursery. His mother was a judge. He had one little sister because each parental unit was only given two children. Jonas thinks his world is perfect.
Jonas is chosen to become the receiver of memories as his life job. He feels a little funny at first. He meets the old receiver who becomes the giver. The man gives his memories to Jonas a little at a time. The good memories make Jonas feel great. The problem is that Jonas has memories that no one else can know or would understand. He starts to feel isolated from the other people.
The people put old people, unwanted babies, and imperfect babies, and people to death. They call it release and think it is a good thing. Jonas has felt death through war. When he watches an infant's death he can no longer tolerate life in the community.
Jonas decides to take a baby that his father has been nurturing at home, named Gabriel, because the child is going to be released. The giver helps Jonas to escape. Jonas and Gabriel escape and go across the river.
The story ends with Gabriel and Jonas hearing music in a distance. No one knows for sure what the future of Gabriel and Jonas will become.