Grey Mask by Patricia Wentworth

In this classic mystery, a young heiress named Margot Standing finds herself in grave danger after overhearing a sinister plot to steal her inheritance. As she seeks refuge, she crosses paths with the astute and compassionate private investigator, Miss Maud Silver. Together, they unravel a web of deceit and betrayal, where masked villains and hidden identities abound. With her sharp intellect and keen observation, Miss Silver navigates the treacherous waters of high society and criminal underworlds, ultimately revealing the truth and restoring justice.