The Devotion of Suspect X by Keigo Higashino,
translated by Alexander O. Smith
All I can say is read this one, but read it all the way through. It's the end that makes it outstanding.
Set in Tokyo, the book starts out with a woman, Yasuko, killing her ex-husband to protect her daughter, so we know who the killer is, who was killed and why. Her neighbor, Ishigami, a brilliant mathematician, decides to help her cover up the murder, so we know who her accomplice is but not why he goes to such extremes to protect her.
I have to protect them, thought Ishigami. He would never be this close to so beautiful a woman ever again in his life. He was sure of that. He had to summon every last bit of his strength and knowlege to prevent any calamity from happening to her. (pg. 36, ARC)
Enter the detectives, a pair of good guys trying to figure out what happened. If it...