Harper Lee (and not everyone will know this because they aren't all as old as we are) is the (female) author of a novel called To Kill a Mockingbird. First published in 1960, her book was a big seller and a big critical success (winning the Pulitzer). It has gone on being both ever since. The 1962 movie, starring Gregory Peck, didn't do it any harm either.
Here's part of the synopsis of the book on Amazon.com:
Set in the small Southern town of Maycomb, Alabama, during the Depression, To Kill a Mockingbird follows three years in the life of 8-year-old Scout Finch, her brother, Jem, and their father, Atticus--three years punctuated by the arrest and eventual trial of a young black man accused of raping a white woman. Though her story explores big themes, Harper Lee chooses to tell it through the eyes of a child. The result is a tough and tender novel of race, class, justice, and the pain of growing up.That tells you enough, I think. It's about growing up (as everyone does, somewhere), and it's about the race issue in the deep south; and it's about justice. Some forty-plus years have passed since I read it, but I seem to remember finding it just a bit too politically correct for my taste (though we didn't use that expression then), and a bit too cutesy and twee. I never took to it.
Most other people did take to it, however. It was the right book in the right place at the right time.
So 'right' was it that, for whatever reason, Harper Lee has never written (or at least published) anything else. She came close, once or twice, but it has never happened. So far.
This, then, is the woman who is the subject of Charles J Shields's biography. He got no help whatever, it seems, from the lady in question: Harper Lee stopped giving interviews in 1965; but he did interview some 600 other people.
I have a horrid suspicion that one of the reasons why Mockingbird has continued to sell is that it is on the reading lists of many a US (and probably UK) Eng Lit course. That is a circumstance which, for a writer, is better than a pension; it's a neat trick to pull off, but I have absolutely no advice on how to do it. As I say, right book, right time, right place. I'm not sure you can plan for that.
Meanwhile you can read Shields's book and try to figure it out.