Here is a summary of the book from Publishers Weekly:
In the winter of 1787, Col. Paul de Saint-Martin, who played a leading role in Mute Witness, travels to England to track down an Irish rogue, Captain Maurice Fitzroy, who’s been accused of raping a young woman of aristocratic birth while visiting Paris. A side benefit of the trip is the opportunity to see Anne Cartier, a teacher of the deaf, whom Paul befriended in the earlier book. Anne is employed as a tutor to the young son of Sir Harry Rogers, a self-made merchant and slave-trader who resides near Bath. Paul and Sir Harry strike up a friendship during a training session of Sir Harry’s prizefighter slave, and Paul soon becomes the slaver’s houseguest at Combe Park. Among the ill-assorted group are Sir Harry and his wife, Lady Margaret, Captain Fitzroy, and Anne and her charge, who bears a striking resemblance to the captain. Also at Bath is the infamous Jack Roach, who is blackmailing several of the city’s inhabitants, perhaps even Lady Margaret herself.
As with Mute Witness, O’Brien fills this book with plenty of twists and turns. He drops Cartier and Saint-Martin into a hornet’s nest of hate and intrigue at Combe Park. O’Brien successfully describes the various conflicts between those who live at Combe Park. In addition, O’Brien meticulously spins all of the intrigue into a spellbinding work.
The different characters are realistic and easy to like and hate as the case may be. For example, you pick up from where the last book left off – hating Jack Roach and his devious plans to wreak havoc on Cartier’s life. Alternatively, you sympathize with Jeff, Sir Harry’s prizefighter slave, on the injustices and abuses he has to withstand in order to live.
Furthermore, O’Brien continues to develop the three main characters – Saint-Martin, Cartier, and Georges Charpentier, Saint-Martin’s adjunct in the Royal Highway Patrol. The love between Saint-Martin and Cartier continues to develop and Charpentier proves once again why he is an indispensable assistant to Saint-Martin.
In conclusion, I highly recommend Black Gold if you have any interest in historical mysteries set in Eighteenth Century Europe.