The Key - Džuničiró Tanizaki

This is the diary of a middle-aged man who is deeply in love with his younger wife, Ikuko. In spite of that love, the pair have grown physically apart, each unsure of the other's desires...until the day Ikuko discovers her husband's diary with its desperate hints of jealousy and voyeurism. Ikuko realises she has found the key to his very soul.