'The Scorpio Races' by Maggie Stiefvater is a mystery wrapped in a myth. On the island of Thisby, every November, deadly water horses emerge from the sea, and riders race them. The secret is how to survive them, both in the race and in general. The protagonist, Puck, enters to save her home, uncovering family secrets and the island's own dangerous relationship with the creatures.
The island is the closed community, its entire culture and economy built around a deadly secret. The mystery is in the nature of the capall uisce, the history of the races, and the personal histories of the riders. It’s a slow-burn, atmospheric mystery of place and character rather than crime, with an incredible sense of dread and beauty.