Yue-ying is the personal servant to the most beautiful and accomplished song girl in the pleasure center, Mingyu. As such, she is not owned by the madam and can come and go as she pleases. Her birthmark (a read cresent on her left cheek) makes her noticeable in town, but her association with the Lotus Palace protects her as well. I liked Yue-ying - she is observant, practical, and a sweet heart without being overly sweet or fake.
Bai Huaung is one of Mingyu's admirers/constant patron (minus the sex part), and the flower prince of the pleasure quarter. His reputation as a flakey, shallow man who enjoys life's pleasures over his responsibilities (such as passing the imperial exams and getting a job); he is fine with this reputation, as it allows him to do his real job without notice. When two dead bodies turn up a connection with Mingyu, Yue-ying and Huaung partner to solve the mystery and fall in love.
The murders are the kind you find in cozy mystery series - light on gory details, the focus is on the motivations and who-dunit. Also the mystery takes a back seat to the romance. There isn't any suspenseful scenes, it is a mellow kind of mystery. I found it was just enough mystery to give the book a fresh take on the historical romance subgenre.