Blue hardcover mystery novel, worn, found near Castillo

Found a blue hardcover mystery novel this afternoon around 2:45pm near the Castillo de San Marcos, tucked under a bench by the information kiosk. It's clearly well-loved—yellowing pages, a bent spine, and a dog-eared corner. Inside the cover there's a red ribbon bookmark and a small lighthouse sticker that might help identify the owner. If you're missing this, I can meet you near the Bridge of Lions or Plaza de la Constitución to hand it over.