I didn't read the book for the challenge, but I remember when I turned that last page and read the ending, I immediately went back to page 1 and started reading again. I had to see all the clues that I'd missed the first time around. (I do admit to liking "gotcha" endings, though, when they're rare and don't feel cheap).
Mystic River is still my favorite Lehane book, though. And to issybird, I'd still suggest trying his detective series. I read them at least 10 years ago, but what I remembered was they were "typical" in that they were procedurals, but "atypical" (at least, IMO) in the way they dug into the human psyche. I always assumed it was because of his and his wife's background in social work.