This is book 3 in an ongoing series. While it can be read as a stand alone, the world building and the overarching story line will make a lot more sense if you start at the beginning, with Kiss of Steel.
Sir Jasper Lynch is the head of the Nighthawks, basically the mostly nocturnal police force of the Steampunk London that Bec McMaster has invented here. All of the nearly four hundred Nighthawks are rogue bluebloods, people who have caught the craving virus by accident in some way (become vampires) and who are not of the Echelon, the nobility who rule the country. After an assassination attempt on the Prince Regent, Lynch is tasked with finding the notorious human revolutionary, Mercury. If Mercury is not brought to justice within a short space of time, Lynch's life is forfeit instead.
Using his enhanced senses and his decades of experience with detective work, Lynch manages to track down Mercury, and is shocked to discover that the revolutionary is a woman. Not only that, she manages to get the better of him by using her feminine wiles to distract him, then she escapes. Now the reason he wants to find her is as much professional as it's personal. Little does he realise that she's much closer than he is expecting.
Rosalind Fairchild needs to find her missing brother, who was involved with the rogue group of revolutionaries who tried to blow up a large part of the Echelon and visiting dignitaries from Scandinavia. She needs to infiltrate the Nighthawks, and gets herself hired as Sir Jasper Lynch's personal secretary. She knows that if he discovers that she is also Mercury, he will arrest her and hand her to the Echelon to be executed, but Rosalind has been trained for subterfuge by the very best, and has no intention of getting caught. She needs to locate her brother, rescue him and then she can go back to ridding the world of all bluebloods.
Rosalind is a humanist, one of the disenfranchised humans who believe revolution is necessary, as the Prince Regent and the Echelon keeps making further demands for blood taxes and humans and mechs (humans who have had to get mechanical prosthetics after injuries) are being treated worse and worse. Few know that Mercury, the infamous revolutionary, is in fact a woman. Recently, a small band of her former followers went rogue, clearly sick of waiting for results. Not content with trying to kill a large group of people with an explosion, they are now working on some sort of weapon that turns bluebloods crazy with bloodlust, slaughtering everyone near them. Rosalind is as determined to stop these rogue revolutionaries as Lynch.
Lynch is the nephew of one of the ruling Dukes on the Echelon council, but was cast off by his family when he refused to fight his cousin in a duel for the rights to be the Duke's heir. Instead he founded the Nighthawks, non-noble bluebloods trained to police the city, solve crimes and bring criminals to justice. What Lynch fears most of all is losing control, and he keeps himself in check, never drinking more than the minimum required amount of blood, never letting himself get emotionally involved. So when Mercury gets under his skin so very quickly, it awakens needs in him he'd almost let himself believe he didn't have. He knows he can never have Mercury, but his newly awakened emotions draw him towards his impudent new secretary, a lovely widow who seems determined to drive him to distraction, even as she tidies up his private life.
Less involved in the larger politics of this world, and more on investigation and police work, My Lady Quicksilver may be my favourite book in the series so far. By now, all the pieces of the world building are firmly in place and McMaster can just let her characters play. The opposites attract story at the centre of this book is delightful. Rosalind is an outlaw, Lynch an officer of the law. She's all about temptation, sensuality and fun, he's all about control, order and work. Neither wants to fall for the other, but they are helpless to control their emotions.
As well as Lynch and Rosalind's romance, I very much liked the supporting cast of Nighthawks in this book. Family is very important in these books, and Rosalind's relationship with her brothers and with her werwulfen friend also adds depth and complexity to the world. Teased in this book, and coming up in the next one, is the romance between two of Lynch's most trusted lieutenants, Garret Reed and one of the few blue blood women, Perry Lowell. I very much liked their banter in this one and hope their book is as satisfying as this one.