This book was written early on (1958) in McBain's writing career. Though he had already started on the famous "87th Precinct" thrillers (the first, "Cop Hater", came out in 1956), this book was a stand-alone work, unconnected to that famous series. It was written when the heavy hand of censorship oppressed writers of all types. You should also remember that it was among his first works, and make allowances for those facts.
The plot. Zach Blake returns to Martha's Vineyard where, a year before, his young wife had drowned. He is intent on finding her killers, for he has received intelligence that his wife was murdered. He sets about the task, gets warned off by various heavies, is implicated in another homicide, and almost gets himself killed in the process. But he keeps doggedly at it and, in the end, works out what happened, who did it, and why. All ends happily ever after (he marries a girl he met on the Island).
The plot is simplistic, the characterisation cardboard; the seasoned reader will work out what happened, and why, long before the protagonist. The book is short - at 123 pages in the Penguin edition, you can read it at a single sitting (I did). So don't expect much. Read it as a historical curiosity, rather than as a thriller per se, and you'll probably get more out of it. And, if you're a writer manque, take heart - even Ed McBain had to start somewhere!