Go Set A Watchman was actually Harper Lee's first book. Her published novel (the ever famous To Kill A Mockingbird) was written at the request of her publisher, who picked the story out of Go Set A Watchman and said "elaborate and publish a whole story about this".
There's been some questions as to why this first-draft manuscript has been published... I'm not going to get in to that, I really want the book to stand on it's own.
First off, for those of you forgetting your High School reading, To Kill A Mockingbird was the story of Scout Finch, her brother Jem, and her father Atticus. When a local black man (Tom Robinson) is accused of raping a young white girl in Alabama, Atticus rises to defend him feeling he deserves to be protected by the law. There's a bunch of other stuff that happens, if you've never read it (or seen the movie starring Gregory Peck) it should go on your reading list.
Go Set A Watchman is set about 20 years later, when Scout (now going by Jean Louise Finch) returns home to Alabama from New York to visit her elderly and sick father. Her brother has died, she has a steady Beau, and Alabama politics are NOT sitting well with her.
Dust ups between the locals and the NAACP have divided the town and she finds that her father, who she once held up as a hero, is on the city council working against African American rights.
You're going to read a LOT of reviews saying "ATTICUS IS AN OLD RACIST", and it's true, he's got some of that going on, but not in the cross burning way you may think. Atticus' reasons for his behaviours and beliefs are very "old guard" - It doesn't mean they're right, but he does have his reasons. His bigotry is built on misinformation and misunderstanding, nothing more and certainly NOT hatred.
"But why isn't he more progressive! In the first book..." yes, I know, we're all heart broken. But it actually DOES make sense when you read it why he does what he does in the first book, in spite of his actions in the second. He's a racist, but not from fear or hatred. He's wrong, but he's also a 72 year old man in Alabama in the 50s... are you really shocked that he has some of that going on?
His character in this novel actually reminded me of Javert from Les Miserables: staunchly for the law to the letter, wanting what is best for the law and by the law to the point of making some short sighted decisions.
What this novel is actually about though, set in and around this conflict, is Jean Louise coming to terms with what all young people must eventually do, confronting the reality of their parents. The Atticus, as told by Scout in Mockingbird from first person perspective, is a hero, a defender, her idol. Conversely, the Atticus as told by Jean Louise (now in a more distant third person) in Watchman is a man with failings, which force her to reconcile not only who he really is, but her own self as well.
This book was great. I really loved it, I found myself raging with Jean Louise (Scout), brought to tears by the rawness of her struggle to finally become her own woman, frustration with history, but also an understanding of view points that, while I don't agree with, I understand a little better.
I think what you'll take away from the book is very personal, no one is going to read this the same way. For me, I'm the same age as Scout, I'm also making my own way and learning things about my parents that I need to reconcile. There's injustice in the world which I'm passionate about, and people I love have viewpoints that seem so backward to me I find it hard to understand them.
I also understand why this book wasn't published in the 50s. It's radical. It's pro African American rights, it's pro Women's Lib, it shakes up the idea of family in a raw way, and delivers a harsh reality to the reader. When I think now about the pro African American, but also pro family and hero worshiping Mockingbird that was written... I almost feel like Harper Lee's voice may have been held back in writing her published work.
My recommendation? Read this book. If you're uncomfortable buying it due to the politics surrounding it, go to your local library and read it. Try not to be offended by the language (it was written in the 50s remember) and do some soul searching, especially in the final two parts.