The Ace of Schemes — Sarah J. Waldock

Toby Davenport did well out of helping his cousin, Lucius, known as Luke, Lord Rokemere, to recoup the family fortune, but he has a mind to increase his own share of what they gained. A genius at mathematics, and also filled with common sense, Toby decides to use half his wealth to gamble with the highest in society, using his skill with probability theory to win more than he loses. He unexpectedly meets Mary Heatherington, the best friend of his cousin Luke's wife, in desperate need of aid. She has had to become a governess and has found herself in dire straits. Toby helps her, but both young people cross a dangerous man, who is out for revenge. Fortunately the patronage of the Prince of Wales, and some good friends stand them in good stead.