You can read my review of parts 1-3 of Stephen King's 11/22/63 here.
What follows are my thoughts on parts four through six and the novel as a whole. I have tried to refrain from including plot spoilers as much as possible but if you're sensitive to that sort of thing, it's probably best you finish reading the novel before checking out my comments.
Having said that, I thought this book was brilliant. King expertly brought the time period of 1958 through 1963 to vivid life for me. Through his simple yet elegant prose, his complex characters, and setting, he managed to create a sense of nostalgia in me for an era before I was born. I identified with protagonist Jake Epping, not because he and I have a lot in common, but because I tried to imagine what it would be like to be told there's a wormhole in time in my favorite restaurant that would allow me to travel back in time for an indefinite amount of time and, although I would age accordingly, when I returned to the present only two minutes would have passed since I left. Then to be charged with the task of picking up where the amiable proprietor of said favorite restaurant left off and attempt to prevent the Kennedy assassination. I can't imagine what I would do. I had no idea, so I had to find out how Jake managed this quest.
Right along side Jake Epping, we plunged down the rabbit wormhole and into 1958 where he financed his operations by using information about major sporting events to lay substantial bets on things where he knew the outcome. He got a job, first substitute teaching and then teaching full-time, and he fell in love. More than anything else, it's the texture of the novel that really brings it to life. This is not just about a man on a mission. This is a novel about a man who travels back in time and fully immerses himself in it. Of course, on some level, at least, he's got to, lest he stick out more than he already does. He meets Sadie Dunhill, and falls in love. This causes at least as many problems as it solves.
There's a nod to conspiracy theorists, too. Before Al, the diner owner who charged Jake with this quest, could feel certain enough about Oswald, Jake was asked to look into the attempt on General Edwin Walker's life, citing that if Lee Harvey Oswald was alone and not convinced by a third-party to assassinate the general, then it was 95% probable that Lee acted alone in the Kennedy Assassination.
The second half of the novel takes Jake Epping (or George Amberson, as he is known in the past) through to the sixth floor of the Texas School Book Depository and beyond. The "obdurate" past is after him with a vengeance.
Does Jake succeed in saving Kennedy and thus providing a better future, or past, depending on how you look at it? I highly recommend reading this fantastic book to find out.