That Matter of Mischief - The Bridgethorpe Brides, #9

Certain she has met every man suitable for marriage to the daughter of an earl, Lady Patience Lumley decides to meet the unsuitable ones. She's not desperate for love and she's only twenty-two, so why not relax and enjoy her Season in London? Crispin Woodhouse doesn't take insult when his good friend asks him to behave like a dullard when he meets the man's sister to convince her she'd be bored married to a man in trade.
But when Lady Patience repeatedly interferes with his investigation, he fears she'll end up in the wrong place at the wrong time. Or worse, he could fall in love.