While there is certainly a mystery in this novel, it is almost besides the point. It is much more about an older man looking back on an important summer of his life, about growing up, friendships and first love (or loss there of).
For the summer between his junior and senior year, Devin gets a job at an amusement park in North Carolina. He is both excited about this job, and a bit nervous about the separation from his girlfriend. It becomes clear over the summer that she has lost interest in him. The park is said to be haunted by the ghost of a young woman who was murdered and left behind in park's one scary ride. While most of the book and the summer concerns Devin's work and his friendship with Erin and Tom over that summer, he is fascinated by the ghost.
Devin ends up staying past the summer, deferring his senior year of college by a year, partially because of his continuing fascination with the ghost and partially to get over his heartbreak. While he remains in North Carolina, he interacts with a young single mother and her terminally ill son. The ill boy seems like many of the other children of King's novels - precocious, with some slight supernatural abilities - I would say he has a bit of the Shining, actually.
It is such a simple story about a young man's coming of age, but what really makes the novel work is that it is told from the perspective of a much older Devin, at least twenty to thirty years later. As a result, that adds a certain amount of wistfulness to the story and yearning for days gone by, friendships gone.
Given that the ghost and supernatural aspect are rather low, this would definitely be a good starter novel for anyone hesitant to try King. As far King fans, if they are anything like me, they don't read King for the supernatural or horror parts of the story, but because he can write characters and stories. Sometimes he may repeat himself a bit but it doesn't mean they aren't good stories.