Apparently: ""The Murder on the Links," the abbreviation "M." is used as an honorific for Monsieur, a French title of respect for men, similar to Mr. in English. It is a common practice in French-speaking regions to use "M." before a man's name as a sign of respect. More info in the Literary significance and reception section of the The Murder on the Links Wikipedia article. In "The Murder on the Links," the abbreviation "M." is used as an honorific for Monsieur, a French title of respect for men, similar to Mr. in English. It is a common practice in French-speaking regions to use "M." before a man's name as a sign of respect."

But "Monsieur" is still used in other parts... Oh well.