Before I Go To Sleep By S. J. Watson

(Used, paperback)
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Christine wakes up every morning in an unfamiliar bed with an unfamiliar man. She looks in the mirror and sees an unfamiliar, middle-aged face. And every morning, the man she has woken up with must explain that he is Ben, he is her husband, she is forty-seven years old, and a terrible accident two decades earlier decimated her ability to form new memories.
Every day, Christine must begin again the reconstruction of her past. And the closer she gets to the truth, the more unbelievable it seems.