This book is fantastic; definitely McDermott's best. Readers who compare it with the movie aren't really being fair, because the movie changes the end of the story completely, though I have to admit that if I hadn't seen the movie, I would never have read the book.
First of all, as an English major and fellow writer, I have to say that the writing here is fantastic! McDermott is poetic, eloquent, and has an uncanny knack for creating believable characters in a time and place that felt all too real. Although I was not alive during the 1960s, McDermott fully realizes not just the protagonists of That Night but all the less central characters as well. Many stories are told in this novel, and in the end the collective storytelling method comments not just on two ill-fated teenage lovers, but on a time, a place, and an entire era that has since passed.
My advice is this: read the book AND watch the movie. Both realize McDermott's vision in totally different ways, though the book feels more true-to-life concerning the nature of first love.