Cheonnune Banhaesseoyo, Heungmangnim!

A prosecutor-turned-royal must race against the clock to solve a string of serial murders… before she becomes the next victim! When an overworked criminal justice employee is suddenly reborn into a novel as the rebellious Princess Charlotte, she also finds herself engaged to the handsome Duke Adler. However, there’s one big problem: according to the novel’s synopsis, Charlotte is doomed to be killed on her wedding night, and the duke is the prime suspect. Now this princess must go undercover to capture the culprit behind the series of sinister crimes… and maybe capture some hearts along the way!

(Source: Tappytoon)

Note: Includes five extra chapters.