The Emperor's Snuff Box by John Dickson Carr

Eve Neill had divorced Ned Atwood. The charge was that he had committed adultery, but Eve had also found him cruel and uncaring. Ned, however, said that she would always love him. Eve lived in the French town of La Bandelette which was one of the most fashionable watering places in France. Across the narrow street from Eve's house was the home of the Laws family. The son, Toby Laws, invited her over for tea, and she immediately felt comfortable with the Laws and especially with Toby who was a banker. The father, elderly Maurice Laws, was a collector of small but valuable art objects. Maurice was an advocate for many humanitarian activities by the British government, but he lived in France to avoid paying British taxes. As time went by Toby asked her to marry him, and Eve accepted.

One evening Ned Atwood used the key to her house which he had lied about losing. He went to Eve's bedroom and begged her to not to marry Toby Laws. Ned said that the Laws family only wanted the marriage to get Eve's money. During their argument, Ned turned to look out the window, and across the street he could see into the study where Maurice Laws worked on his collection. Ned and Eve could see that someone had attacked Maurice Laws and brutally murdered him. Eve insisted that Ned leave before the police arrived at the Laws house. He returned her key and started down the dark staircase. He tripped over a lose stair and fell down the staircase. When Eve reached him at the bottom she found him alive, very dazed, and bleeding. Some of the blood got on her hands and the white negligee that she was wearing. She got Ned up and out of the backdoor which then closed locking her out. She had Ned's key to the front door in her pocket so she had to sneak around her house to get in the front door because the police had arrived across the street.

One week later, the police decided to arrest Eve for the murder of Maurice Laws. Her servants had shown the police the blood stained negligee and had witnessed her sneaking back in the front door. The police had also found on her negligee a tiny fragment of Napoleon's snuffbox which had been smashed on Laws' desk. Ned Atwood, the one person who could back up her story, was unconscious in the hospital with a concussion due to his fall down her steps.

At this point, I will leave you worrying about how Eve got out of this dilemma. Adding anything else would be a spoiler. I will say that she did receive a great deal of help from a distinguished  English doctor named Dr. Dermot Kinross who was very instrumental in finding the murderer.

This book is by the great master of mystery John Dickson Carr. It was published in 1942, and is still available in paper format and as an e-book.