I recommend Blood from a Stone, by Donna Leon. If you look at the photo on the opening screen of her website, you’ll immediately see what I liked most about this novel: it makes the reader feel that s/he’s in Venice. For some reason I seem to be picking mysteries with a well-developed sense of place lately, and this book is no exception. You can see, hear, and smell Venice as you read it.
That’s #1, but Blood from a Stone has other strengths as well. For one, it deals with issues of prejudice and illegal immigration, familiar to us here but pleasantly jarring in the context of northern Italy. Also, the characters and plot are three-dimensional. The teenagers in the book are a bit too much stereotypically teenagers, but I suppose many teenagers are — and the recurring presence of the protagonist’s family life humanizes him in a way that nothing else could do. The book is somewhat leisurely, so don’t read it if you want a fast-paced mystery. Pick a time when you can be a leisurely and thoughtful reader, and you won’t regret it!