Or third in the Olivia series, depending on how you look at it.
For those unfamiliar with the series, the Saint-Germain series is a series of historical "horror" novels (although the horror element is tenuous at best, based purely on the fact that the main characters are vampires, and "vampire fiction" is considered a subgenre of "horror fiction"; actually, "historical romance" is closer to accurate) in which the main character is the vampire Saint-Germain, who has lived as a vampire since roughly 1500-2000 BCE. In this book, however, as in the previous two, the main character is Atta Olivia Clemens, who as a lover of Saint-Germain's became a vampire when she died, back in the Rome of the Emperor Nero (in the third book of the series, "Blood Games".)
This book is set in France during the reigns of Louis XIII and Louis XIV, which is the period during and slightly after the time of Cardinal Richelieu of "The Three Musketeers" fame. The "d'Artagnan" of the title is based on the historical Charles d'Artagnan on whom Dumas based his hero, not on the fictional hero himself.
In some ways, this book is better than the two previous books focusing on Olivia; my major complaint about them, that Olivia's vampiric powers were downplayed too severely, does not apply to this book. But I have a very major problem with EXTREMELY major plot points happening offstage, and the reader being informed of them after the fact and given an insufficient description of events to follow the action. This was badly done, and is a major part of my failure to rate this book more highly. Also, the editing did not seem as tight as in the previous entries; far too many typos and incorrect word usage (being "adverse" to something, rather than "averse", etc) managed to slip through. I hope this trend doesn't continue into the later books in the series.
This is an enjoyable read, better than some in the series, certainly not as good as "Tempting Fate", the fifth book in the series.