The only good thing I can say about this book is that it uses an interesting storytelling device by checking in with characters once a year over a long period of time.
The plot. Was. Awful.
Seriously, don't waste your time waiting for it to get better. It won't.
The only person with a moral compass is the lead character's brother, who the writer tends to mock because he is married multiple different times. The only people who seem happily married are the main character's parents, and they get killed off. Does the author think marriage is antiquated and meaningless? Feels that way.
The two main characters decide to carry on an affair for 28 years, but only for one weekend, once a year. Why, do you ask? What is such an insurmountable obstacle that they can't be together and be happy? I can't give you a good answer. I read the whole book, and the most it's ever explained is that the woman doesn't want to leave Nantucket. The man seems willing to move there, so never understood what the problem was.
There are very romantic parts, some interesting characters with a lot of levels to them, and a couple good plot devices. But not worth the completely unrealistic and pretty horrible premise of the plot. Oh yeah, then it feels like a Nicholas Sparks novel at the end, where the woman dies. I would say that's a spoiler alert, but you know it from the beginning. And the children of the two main characters either fall in love, or start their own long term same-time-next-year fling ridiculousness. Awful.