A Midsummer’s Equation by Keigo Higashino
Higashino might be one of my favorite authors. A Midsummer's Equation is the fourth of his mysteries I've read I've read. It's the third Detective Galileo translated into English but the 6th in the series. It doesn't matter; the ones I've read definitely stand-alone.
As the blurb above says, Manabu Yukawa is at a run-down resort town to attend a conference when, surprise, surprise, someone gets murdered. Yukawa is a physicist - good at observing, logical, thoughtful, quiet. He's that character that knows what's going on but isn't going to brag about it. We also get to see his more caring side here. He becomes friends with a boy who is also staying in town and they have some very good scenes together. His concern for the boy is what pulls him into the case, and his natural tendency to get involved in mysteries - he is the series' star.
A lot of mystery blurbs talk about a surprise twist, but Higashino actually lives up to...