Late for the Wedding by Amanda Quick

Private investigators Lavinia Lake and Tobias March are hot on the trail of a murderer for hire . This time a macabre murderer appears to be copycatting two killers who left behind memento mori rings at the scenes of their crimes. The original Memento Mori Man murdered for hire decades earlier, taking on assignments only if he thought the target thoroughly deserved death. Three years before the start of this novel, the same "ghastly signature" was employed by Zachary Elland in tribute to his predecessor. Elland, March's fellow spy (during the Napoleonic wars), committed suicide after March unmasked him, so when a memento mori ring turns up on the doorstep of Elland's rich lover, Aspasia Gray, March is not surprised when she seeks him out at Lord Beaumont's country-house party. Lake, however, is dismayed, since Gray wants more than just March's investigative skills. A murder is promptly committed at Beaumont's estate, and Lake and March launch their investigations, centering on a certain unstylish blonde wig and a trio of dangerous, dowagers.