The Magistrates of Hell by Barbara Hambly

 Set in Peking, 1912, James Asher and his lovely wife, Lydia, are on the trail of dangerous supernatural "others" who prey upon both the living and the undead. Hambly wastes little time getting the action moving - during the opening scene of an engagement party at an embassy, the young bride to be is strangled, seemingly by her beau, the son of one of Asher's old colleagues, Hobart. Don Simon Ysidro is also in attendance at the party, though he mysteriously disappears just before the murder is committed, after James has queried him for just enough information about the others to whet his, and we gentle readers', appetites.

Asher agrees to do what he can to exonerate his old friend's son, Ricky, which turns out to be a minor, and mostly ignored, piece of the plot, wrapped up in a sentence or two at the end of the novel. The backstory to why he was framed for a murder is slightly more integral, but the main story is about how James, Lydia, and assorted unlikely allies band together to eradicate the undead menace before the powerful players in Peking are able to make use of them to gain even more power.

I don't recall Lydia ever displaying any great sense of adventure in earlier stories, so it may be that Hambly is doing a bit of character development with her. When their adversaries attempt to sideline James by accusing him of treason, he fakes his own death, and she is the only soul who knows he is alive. She does a splendid job of acting the bereaved widow, while pressing forward with the investigation on her own.

A good conclusion and a not-unexpected plot twist to wrap up this tale.