Creativity Challenge: The Fourteenth Letter by Claire Evans

Going off the Cover
It is a murder mystery set in the 1800s that involves a wedding. The bride is going to either be victim or killer. and the groom is a swashbuckling drunk.
Goodreads Synopsis
One balmy June evening in 1881, Phoebe Stanbury stands before the guests at her engagement party: this is her moment, when she will join the renowned Raycraft family and ascend to polite society.

As she takes her fiancé's hand, a stranger holding a knife steps forward and ends the poor girl's life. Amid the chaos, he turns to her aristocratic groom and mouths: 'I promised I would save you.'

The following morning, just a few miles away, timid young legal clerk William Lamb meets a reclusive client. He finds the old man terrified and in desperate need of aid: William must keep safe a small casket of yellowing papers, and deliver an enigmatic message: The Finder knows.

Did I Get Anything Right?
The bride/fiancee is indeed the victim. Beyond that and the year which was provided on the cover, I didn't get anything else spot on. It remains to be seen if the potential groom was a drunk of any kind or not.