Why did sei shonagon name her book the pillow book?
The original name in Japanese is 枕草子 (まくらのそうし) where 枕, まくら (makura) means 'pillow' and 草子, そうし means 'text, novel, notebook' or something along those lines. I think what Sei Shonagon ment by the title was to suggest her work was somehow the result of her contemplation/ cogitation/ thinking process and analysis of events gone throughout the day as she would go to bed, lay her head on the pillow before falling asleep. I suppose you could interpret it along this line. I obviously take it to be a metaphor, but, she might have wanted to suggest that sort of evaluative, contemplative state of mind in which one recollects ones experiences and, in result of this overview, can judge and reflect their implications and value and all that.