I really enjoyed the first two thirds of the book, then after that it seemed like the author had written himself into a corner with his complex mystery. His solution was to throw in a complete tonal shift, "twists" in characters that were never set-up and then the kitchen sink for good measure. The main plot motivation, when it was revealed, is disgustingly hideous and the descriptions are nauseating. I stuck with it, figuring that the pay off really hinged on it. But it doesn't. The finale was rushed, underwhelming and disappointing due to what had come before. What I would call the major antagonist is left almost completely out of the epilogue leaving a huge hole in our sense of completion. Other characters are left behind and we feel angry at those loose threads too. But perhaps most annoying about the ending is that it is just a huge anticlimactic dud for what was set-up.
Overall, it just felt like I was reading a first draft, I didn't mind going back to the past then back into the present, but towards the end it didn't feel like these flashbacks were adding anything at all to the story. They were just padding. I can't really recommend the book despite its incredibly strong opening and well crafted mystery for the first 150 pages because the descent afterwards is far too steep to not be a waste of your time.