Deep Lake Mystery

Deep Lake, in Wisconsin, had a curious and sinister charm. By day it was a charming resort for summer visitors, but by night its character took on sinister depth like the swirl of its own waters. The murder of Sampson Tracy was purely the strangest of all murders. He died of a nail driven into his skull, and round his body were found flowers, fruit, a feather duster, and other seemingly meaningless articles.
Find the motive and you find the criminal. But several people have motives which may have led them to the deed. Which one did it? Why did the murderer decorate his victim with those gruesome inanities? And how was the crime committed in a sealed room?