Death Wears a Mask/Mr. Crook Lifts the Mask — Anthony Gilbert

DANGER FROM THE DARK
Wandering in the night, May had seen something she didn't understand--until a girl's murdered body was found in the same spot. Then she knew what she had seen--and she knew, too, that the murderer had seen her, and that her turn would come next....

Then Arthur Crook came into her life like a comforting whirlwind, aggressively ready to protect her and expose the killer. But the case took a strange turn and brought Crook a new client--the suspected murderer!

Original Title: Death Wears A Mask