Jane and the Hidden Hoard — Sarah J. Waldock

Jane and Caleb are hired to work outside London; by the rules of Bow Street ‘when a gentleman writes, the gentleman pays'. At stake is the whole livelihood of Sir Colin, a man from whose custody were stolen a number of jewels belonging to other people, and Lloyd's of London won't pay the insurance. The thief's accomplices have been arrested but they refuse to talk, or reveal his identity, and Jane and Caleb must undertake a masquerade to take their place in Sir Colin's household. Jane's and Caleb's primary task is to find the jewels. Their secondary task to uncover the thief, and lift suspicion from other members of Sir Colin's family. The stakes are raised, however, when the fire-raising thief shows himself to be lethally ruthless…