Lady Knight by Amalie Howard

The daughter of a duke defies the rules of London high society with the help of the resident rake—who just so happens to be her brother’s best friend—in this fun teen historical romance from the USA Today bestselling author of Queen Bee.
Lady Zenobia “Zia” Osborn, a duke’s daughter, is frustrated that her entire life has been predetermined. What good is skill or intelligence if one is forced to suffocate it because of one’s sex? She’d much rather make her mark on the world than bat her eyelashes for the ton.
Zia only comes alive in the Lady Knights, a clandestine social club for rebellious girls. In it, she is free to compose music, fence, read controversial literature, and save orphans from destitution by any means necessary. Aside from her closest confidantes, no one knows about Zia’s indecorous other life. . . .
Until Mr. Rafi Nasser, a rogue with secrets of his own, finds out. Shocked yet intrigued by his best friend’s younger sister, he agrees to keep Zia’s secret—if only to help her avoid utter scandal.