This is a good request, but it is not something easy to do. To distill a book (a great one at that) in a sentence is nearly impossible. Moreover, people will differ greatly in their opinions. To be honest, every time I read it new ideas come into my mind and I see the book in a different way. In light of this, allow me to give a few sentences.
To Kill a Mockingbird is a coming of age story of two children growing up in a racially charged southern town.
Here is another attempt.
To Kill a Mockingbird is novel about the importance of courage and integrity to do the right thing in the face of great obstacles and opposition.
Here is still yet another attempt.
To Kill a Mockingbird is the reflection of a woman who grew up in a racially turbulent time and her ability to choose a better path through the guidance of her father.
In To Kill a Mockingbird by Harper Lee, the story follows the journey of two children who learn about justice and innocence through their attorney father who tries to prove the innocence of a black man falsely accused of rape, and their next door secluded neighbor.