Trace Elements by Donna Leon
I've read many of the Brunetti series, but generally out of order. I like Brunetti. He's a reader and a thinker. He loves Venice, but isn't fond of the heat and the tourists. He's doesn't approve of the corruption but realizes that to some extent it's what makes Venice run.
In Trace Elements, Brunetti and Claudia Griffoni are called to the bedside of a dying woman. She tells them that her husband was killed because of the "bad money," but dies before she can tell them more. With no real evidence of a crime, Brunetti and Griffoni start investigating, feeling they owe it to the dead woman. Their clues take them to a water testing lab where perhaps not everything is handled as it should be.
The plot was just complicated enough. Of course, we know that it was a murder, it always is in these books, and we know that with Brunetti's perseverance and intuition, he and Griffoni will find...