If you have not read The Giver, Gathering Blue, and The Messenger - do not continue to read this. You will not understand anything I have to say, plus it will spoil the first three if you read Son first.
Based on the title, you can probably guess what this book is all about. This is Claire's story, the biological mother of Gabriel, who in the first book, The Giver, is saved from "release" by 12-year-old Jonas. At the end of the first book, you are left wondering. You learn a little more in the next two books, and finally in Son, it all comes together.
Lois Lowry is an awesome author, and Son is being marketed as the conclusion to the quartet. However, I was left a little disappointed. After all this time (remember that The Giver was first published in 1993) I wanted more. Even though Claire is an interesting character that we knew nothing about in the first book, there was little to explain why the society did what they did to her son. Conclusion generally means "the end" so I guess we will never know. Even after 20 years of waiting...