The Risky Wager of a Masked Lady by Hanna Hamilton

Whatever souls are made of, his and hers were the same…
Lady Marjorie Acton never expected to manage to convince so many people that she is a man.
When her father, the Marquess of Mortham, announces that she is to marry a gentleman nearly thrice her age, she decides to rebel against him. Under the cover of night and a servant’s clothing, she leaves for the one place that always felt like home: the horse racing tracks.
Evan Sedgewick, Earl of Lanercost and successful breeder of racing horses, can barely believe his eyes when his newly recruited rider emerges victorious from the race, exhibiting unprecedented talent.
But when an accident nearly claims Marjorie’s life, not only is her cover blown, but it also becomes apparent that Evan has a very deadly enemy. One that will stop at nothing to get what they want: Evan’s life and the Acton family’s greatest secret…