Trent's Last Case (Unabridged)

This is one of a series of EC Bentley novels featuring the highly erudite artist qua reporter / detective, Philip Trent. In it, Trent is sent to a charming English seaside village to cover the murder of Sigsbee Manderson for a large London newspaper. The victim is an unpopular and extremely powerful financial tycoon, who is murdered virtually within sight of his own house, at a time when it seems impossible that anyone there - to say nothing of all of its more than half dozen inhabitants - could have failed to see or hear the crime being committed.
As Trent pokes around, attention is focused on Manderson's extremely troubled marriage, not least because Trent himself falls in love with Margaret Manderson, the widow of the murdered man. At the same time, Trent himself considers her to be at least complicit in the crime for much of the novel. The plot cannot be described further without spoiling the punch-line, as it were. Indeed, to the really clever detective fiction-lover, this is already almost saying too much.
Trent's Last Case was on the "ten best" list of Rex Stout, author of the famous Nero Wolfe mysteries. Like Stout, Bentley has a fondness for complex plot twists of the "boxes within boxes" variety.