[Looks a little surprised, especially as this is the first time she's heard anyone else mention the book than the Musketeers themselves, always in a joking tone, but... the deduction still earns Sherlock a smile.]
You have quite the skills of observation, don't you, monsieur? But you're right -- even when they can hardly be called my Musketeers. They're the King's Musketeers; I am only friends with them.