The Less Dead (Denise Mina)

When Margo goes in search of her birth mother for the first time, she meets her aunt, Nikki, instead. Margo learns that her mother, Susan, was a sex worker murdered soon after Margo's adoption. To this day, Susan's killer has never been found.
 

Nikki asks Margo for help. She has received threatening and haunting letters from the murderer, for decades. She is determined to find him, but she can't do it alone...