Blue hardcover mystery novel, worn spine

I lost my blue hardcover mystery novel this afternoon around 3pm near Waterstones on North Street, Brighton, by Churchill Square. It's a hardcover edition with a navy cover and a silver-embossed title along the spine. Inside, there's a bright yellow ribbon bookmark stained with coffee, and on the inside cover there's a small sticker with a cat drawing. It belonged to my grandmother; it was her gift to me, and it feels irreplaceable. If anyone found it, please get in touch—I'd be forever grateful, I really need it back.