"M." is an honorific applied to people in the Hyperion series of novels by Dan Simmons.
This title replaced "Mr." "Miss" "Ms." and "Mrs." (probably "Madame", "Monsieur" "Mademoiselle" and "Master", too), and I found it strangely pleasing to both the eye and ear. It added greatly to my sense of immersion in the books.
Examples: "M. Gladstone", "M. Weintraub", etc.