The Perks of Loving a Wallflower by Erica Ridley

Fans of Bridgerton will love this Regency romp in which a proper Society miss recruits a very improper lady investigator in a quest for vengeance, only to find love instead.

As a master of disguise, Thomasina Wynchester can be a polite young lady—or a bawdy old man. She’ll do whatever it takes to solve the cases her family takes on. But when Tommy’s beautiful new client turns out to be the highborn lady she’s secretly smitten with, more than her mission is at stake . . .
Bluestocking Miss Philippa York doesn’t believe in love. Her heart didn’t pitter-patter when she was betrothed to a duke, nor did it break when he married someone else. All Philippa desires is to decode a centuries-old manuscript to keep a modern-day villain from claiming credit for work that wasn’t his. She hates that she needs a man’s help to do it—so she’s delighted to discover the clever, charming baron at her side is in fact a woman. But as she and Tommy grow closer and the stakes of their discovery higher, more than just their hearts are at risk.