The Pillow Book can be translated as Makura no Sōshi, literally "notes of the pillow"
scholars understand this as "a generic term to describe a type of informal book of notes which men and women composed when they retired to their rooms in the evenings and which they kept near their sleeping place, possibly in the drawers of their wooden pillows, so that they might record stray impressions"
this "is the precursor of a typically Japanese genre known as zuihitsu ('occasional writings,' 'random notes') which has lasted until the present day and which includes some of the most valued works in the country's literature"
Ivan Morris, in translator's note to Columbia edition of Shonagon's writings