Wilde, Darcie. Useful woman mystery

"Rosalind Thorne may not have a grand fortune of her own, but she possesses virtues almost as prized by the haut ton: discretion, and a web of connections that enable her to discover just about anything about anyone...Mrs. Gerald Douglas, nee Bethany Hodgeson, presents Rosalind with a particularly delicate predicament. A valuable pearl necklace has gone missing, and Bethany's husband believes the thief is Nora, Bethany's disgraced sister...The intrigue swirling around the Douglases includes fraud, forgery, blackmail, and soon, murder. And it will fall to Rosalind, aided by charming Bow Street officer Adam Harkness, to untangle the shocking truth and discover who is a thief--and who is a killer"--