The publication of Go Set a Watchman was a surprise for a number of reasons, the biggest of which was that this novel by Harper Lee was actually the original manuscript for her classic and beloved novel To Kill a Mockingbird. The second reason is, obviously, the decision for the book to be published at all, after so many years.
Go Set a Watchman is a book that in one sense needs to be viewed very much in its historical context--its not a sequel to To Kill a Mockingbird and nor it is a prequel. What is it, is an earlier draft of the same novel, one that was not accepted for publication.
And that said, it is still a brilliant book. Go Set a Watchman is uncomfortable reading at times, but I strongly suspect that it was supposed to be. It opens with an adult version of Scout Finch, now better known as Jean Louise Finch, returning home to Maycomb from New York (where she has been working,) to visit her ailing father Atticus, her aunt and her sort-of beau, Henry. Through the combination of flashbacks and a present day narrative, we see that Jean Louise's visit home is becoming increasingly uncomfortable as she discovers some home truths about her father--a man whom she always, unquestioningly looked up to--and tries to reconcile with the fact that he is not the man that she believed him to be and that some of their core beliefs are different. Some reviewers have argued quite strongly that Atticus is a racist, I feel that there is sufficient evidence in the book to suggest that he is simply trying to coexist within his small town and living in a way that is neither politically correct nor easy for modern readers to understand--for example Atticus joins the KKK just so that he can find out who is in it, and does not silence others with their racists ramblings, though it is never suggested that he inherently agrees with everything that it is said. Or if you want to look a bit deeper at some clever foreshadowing, look at the flashback scene where Atticus allows the minister who is eating a meal with the family to lay a guilt trip on the children for playing a game in which they were baptising one another, and later hides away, and then privately laughs about the ministers stupidity later. That is the kind of man Atticus is. It is not, however, the kind of person that Jean Louise is, and much of the novel examines how Jean Louise reconciles with the fact that she is a different person to her father, and that she has her own beliefs based on her own experiences. She sets her own conscience, hence the title. At the heart of the book is the message that who we think our parents are when we are children, and who we discover them to be as adults can be quite different.
A little dry in places, and with a third person narrative that occasionally shows the protagonists confused thoughts, Go Set a Watchman is a book that requires a lot of careful thought, rather than knee jerk reactions. The author places a significant amount of trust in her readers to pick up various nuances and it also needs to be viewed in the context of when it was written, where it is set and the reasons that the author had for writing it.