Widow's Walk by Robert B. Parker

When Nathan Smith, 51, is found in bed with a hole in his head it's hard not to imagine his young bride as the one with the finger on the trigger. Even her lawyer thinks she is guilty. But given that Mary Smith is entitled to the best defence she can afford - and thanks to Nathan's millions, she can afford plenty - Spenser is hired to investigate Mary's bona fides. Her alibi is flimsy - she claims she was watching TV in the other room when the murder occurred. But the couple were seen fighting at a high profile cocktail party earlier that evening and the prosecution has a witness who says Mary once tried to hire him to kill Nathan. What's more she is too pretty, too made-up, too blonde and she sleeps around - just the kind of person a jury loves to hate.