Hatchet Island is a great addition to the series, but it would be enjoyable as a standalone as well. The main character, Maine game warden investigator Mike Bowditch, stops on a sea kayaking trip at a research station on an island sanctuary for endangered seabirds. All is not well on the island. There’s a good mystery and engaging scene descriptions. You can enjoy it as a standalone but I do think it would be most enjoyable if you read the series in order as there are some changes for Bowditch that will be more meaningful for series readers.