Blue Penguin Classics mystery novel

I lost my blue hardcover mystery novel yesterday around 3pm. It's a Penguin Classics edition, about 320 pages, with a blue cover and gold lettering. There's a small dent on the top corner and a coffee ring along the bottom edge. Inside, a fox-shaped bookmark marks page 142. I was in Falls Park in Sioux Falls near the pedestrian bridge by the river overlook when I realized it was missing. This book has sentimental value to me; my grandfather introduced me to it, and I really need it back. If anyone finds it, please reach out—I'd be grateful to get it returned.