What Darkness Brings by C. S. Harris

London, September 1812. After a long night spent dealing with the tragic death of a former military comrade, a heart-sick Sebastian learns of a new calamity: Russell Yates, the dashing, one-time privateer who married Sebastian's former lover Kat Boleyn a year ago, has been found standing over the corpse of notorious London diamond merchant Benjamin Eisler. Yates insists he is innocent, but he will surely hang unless Sebastian can unmask the real killer. For the sake of Kat, the woman he once loved and lost, Sebastian plunges into a treacherous circle of intrigue. Although Eisler's clients included the Prince Regent and the Emperor Napoleon, he was a despicable man with many enemies and a number of dangerous, well-kept secrets--including a passion for arcane texts and black magic. Central to the case is a magnificent blue diamond, believed to have once formed part of the French crown jewels, which disappeared on the night of Eisler's death. As Sebastian traces the diamond's ownership, he uncovers links that implicate an eccentric, powerful financier named Hope and stretch back into the darkest days of the French Revolution. When the killer grows ever more desperate and vicious, Sebastian finds his new marriage to Hero tested by the shadows of his first love, especially when he begins to suspect that Kat is keeping secrets of her own. And as matters rise to a crisis, Sebastian must face a bitter truth--that he has been less than open with the fearless woman who is now his wife.

This book is part of the "Sebastian St. Cyr" series.
Here are some other books from this series: