Book cover has a drawing of a male detective in a jacket standing in the middle of the cover facing the reader maybe wearing a hat too in a dark cobble street with a lamp lit?

It was released post 2000s pre 2018 it was a detective story and I can't remember much from it except there was a scene where they found a shark in an abandoned aquarium? And I think they main plot point had something to do with a diamond or jewel? I remember that there were multiple books because the first ended on a cliff hanger.