The Wallflower Manifesto by Ava Devlin

Millie Yardley has never fit the mold.
Older than her newly married sister, curvier than society deems fashionable, and perfectly aware that no one expects her to do much beyond fade quietly into spinsterhood, Millie decides to try something new.

When she accepts the position of lady’s companion to the formidable Dowager Countess Bentley, Millie returns to a London that looks very different than the one she remembers. Suddenly, there seem to be many ways to be a woman. Bold ways. Complicated ways. Ways no one ever told her were allowed. Her pen can hardly keep up with every stunning new thing she sees, all of it a revelation for girls like her: the wildflowers in a sea of English roses.

Abe Murphy isn’t stalking anyone! He’s investigating.
Former Bow Street Runner turned private agent, Abe is looking into a case on behalf of his irritating housemate, London’s most notorious reformed scoundrel. The last thing he expects is to get caught by a sharp-eyed wallflower with a devastating mind and no patience for nonsense.

To distract her, he ropes her into an old jewel theft investigation he long ago gave up on. But Millie doesn’t give up. Not on a mystery, and not on a challenge like Abe Murphy. As their reluctant partnership turns into something far more intimate, Millie’s private writing takes on a life of its own. When her manifesto is published without her consent, it sets off a chain of events that could destroy reputations—or save lives. And Abe may be the only man willing to stand beside her when it matters most.