Or fourteenth, if you count "Out of the House of Life", a spinoff novel primarily about the character of Madeline de Montalia, a vampiric "childe" of Saint Germain, but also including some flashback scenes featuring an early Saint Germain.
Or seventeenth, if you also include "A Flame In Byzantium", "Crusader's Torch", and "A Candle For d'Artagnan", a spinoff series about Olivia Atta Clemens, an earlier offspring.
Throughout the series, the best part of these novels is the character of the count Saint-Germain himself; he is an unmitigated hero, not the anti-hero that one usually sees in vampire novels, and that's a fascinating change of pace. He always explains that he wasn't always the urbane, elegant, even-tempered, kind and sensitive individual that he is now; four thousand years ago, when he became a vampire, he was a typical ravening beast, but he outgrew it. This is a marvellous and original perspective on vampirism, and a delightfully optimistic outlook on humanity: that given sufficient time, ANYBODY can grow up, even a bloodthirsty creature of the night.
As a result, what we have in this series is a series of historical novels, set at various points along the very long time-line of Saint Germain's life. We generally see very little of other vampires, other than occasionally seeing those who Saint Germain has made vampires in previous books. (Generally, we see even these only in their correspondence with Saint Germain; I cannot remember any book in which we see more of them than this except for "Tempting Fate", in which we see quite a bit of Madeline de Montalia, and one short story in the collection "The Chronicles of Saint Germain", in which we see the count in conflict with a more traditionally minded vampire.) This book is the exception to that rule, as well as the rule that each book covers a "point" on his time-line. This book is told in three sections, each a separate point of its own: one in the seventh century, one in the eighth, and one in the twelfth. This divergence from form is necessary in order to show the fact that interactions between vampires are by necessity very long-term things. The main conflict in this book is between Saint Germain and a woman who he makes into a vampire early in the book, and who refuses to accept him as a mentor on how to manage her new life and the powers that go with it.
This was an interesting change of pace, but resulted in a novel that was long on sub-plots, but had less primary plot than it might have. In addition, it made it difficult to see Saint Germain interacting with mortals as one normally would, because by definition, none of the mortals survived more than a third of the book. We did see interations, but there wasn't time to develop them as there normally would be, and that left them feeling rushed. Still, the loss of that aspect of the books was easily compensated for by the novelty of seeing other vampires, vampires who acted as vampires are expected to, for a change.
Not one of the best in the series, but far from the worst.