The Necklace Affair (Captain Lacey Regency Mysteries #4.5)
London 1817. Captain Lacey agrees to track down the missing necklace of a society matron and prove the innocence of her maid, who has been arrested for its theft. He recruits Lady Breckenridge to infiltrate the matron's household, and finds himself competing with the underworld criminal, James Denis, for the necklace's retrieval.
This is a ten-chapter (25,000 word) novella.