In the Midnight Hour. Only Murders in the Square, #1

The murder of the Viscount Northwood's neighbor has finally given the idle bachelor something to do. When he suspects the enigmatic widow across the square of the crime, he employs the investigative skills gleaned from his Bow Street Runner twin brother. However, the anonymous broadsides circulating around London depicting every aspect of the murder are foiling his investigation. Upon discovering his prime suspect is the author of the publications and not the murderer, North partners with her to solve the crime before the killer strikes again.
After finding the killer, they continue to work together to solve murders that keep happening only in their cozy Regency London square. The idle heir to an earldom and an enigmatic widow, whom he initially suspects is a killer, team up to solve murders that keep happening only in their cozy Regency London square.