"Y is for Yesterday" is a mystery novel by Sue Grafton, part of the popular Kinsey Millhone series. The story follows private investigator Kinsey as she delves into a cold case involving a murder that occurred years earlier. As she uncovers secrets, she faces challenges from both the past and present. Set in the 1980s, the book explores themes of memory, regret, and the impact of unresolved issues. Kinsey's determination to find the truth leads her to confront dangerous individuals, making for a suspenseful and engaging read.