Swimming Lessons by Claire Fuller

Ungrid Coleman writes letters to her husband, Gil and leaves them in books in the library. When she finishes her final letter, she disappears. Her daughter, Flora comes home twelve years later to care for her ailing father. She has been searching for her mother ever since, never believing that she has died. The story goes back and forth between the letters and Flora struggling to put the past behind her. A great, suspensful read.