Blue paperback mystery with lighthouse cover

I lost my blue paperback mystery yesterday around 3pm outside the little bookstore on St. George Street, just off the Colonial Quarter, here in St Augustine. It's a softcover, about 320 pages, with a bright blue cover and a white lighthouse illustration; the spine is a bit creased and there's a coffee stain along the bottom edge. A thin red ribbon bookmark sticks out from page 214, making it easy to spot. It was a gift from my grandmother, which is why it matters—sentimental value and I really need it back. If you happened to pick it up, let me know.