I'm another who thought the ending was a con job. I hate cheap gotcha moments. Because after the shock, the immediate reaction is a huge so what? Everything that's happened in the book is negated and it's hard to care. I thought Lehane was making some points about the various schools of psychiatry and treatment, but since the exposition counted for nothing at all, there goes that.
In justice to Lehane, I think it was fair enough. There were enough clues and shame on me for not seeing the anagrams--they usually jump out at me. I'm embarrassed to admit that I got so far as thinking that Chanal and Laeddis were peculiar names but didn't take the final step.
I'm disappointed for another reason, although it's not the book's fault. I've always thought that someday I'd give Lehane a try for the local color. Now it'll be a long time before I try one of his traditional detective novels set in Boston.