The Grand Sophy by Georgette Heyer

When the irrepressible, over-six-foot-tall Sophy Stanton-Lacy is left in the care of her maternal aunt, Lady Ombersley, she arrives in London like a whirlwind, turning her respectable, debt-ridden relatives' lives completely upside down. Rather than conforming to the demure expectations of Regency high society, Sophy relies on her sharp wit, bold independence, and sheer financial resourcefulness to resolve her cousins' romantic crises and untangle the family's financial miseries. Her unconventional, high-spirited schemes routinely clash with the strict, autocratic authority of her eldest cousin, Charles Rivenhall, who is stubbornly betrothed to a joyless prude. Through carriage races, a dramatic moneylender confrontation, and masterful matchmaking, Sophy ultimately coaxes Charles out of his tyrannical shell, transforming their fiery battle of wits into an unexpected and deeply satisfying love match.