Hearts in Atlantis.
I held this one off for a couple of years, not expecting it to be my cup of tea, nevermind rending my heart. I just wanted a happy ending for those two kids, now grown up, but King--correctly--refused to give me that saccharine and false conclusion. Instead he served me a cold dish of truth and realism.
And he warned me throughout the book that it was coming, but I still didn't see it. They discuss the unhappy ending of Lord of the Flies at length, and Ted mostly shrugs, stating flatly: "It's an ending."