The sleepwalker
From Goodreads: "Marian Tansey appears to be living a happy life. She has great friends, a job in a thrift shop, and she has just bought a new car. She may even be falling in love with Dick Lang, who sold it to her. She could be on top of the world, but there are a few clouds in the sky. There is a mystery surrounding the car. It has been 'borrowed' during the night by someone unknown. But most of all there's the frightening fact that, although she hasn't admitted it to any of her friends or colleagues, Marian lost her memory a year or two ago and has no idea who she is. Then, there is a murder . . ."