A pillow book is a collection of things written down by one of the upper class courtiers -- at least posing as intended only for the writer -- and this is a charming look into Heian court. (I recommend The World of the Shining Prince first because the context is useful. Less need to refer to the notes.)
It's a hedonistic, shallow, snobbish life -- entirely from the point of view of a member who takes her contempt for the vulgar commoners entirely for granted -- but touched with a keen awareness of beauty and aesthetics. She enumerates lists of things, anecdotes (featuring herself, or not), and observations. The lively love affairs of the court, in particular, get portrayed in both anecdotes and rules; she observes it's much easier in your parents' house than in your in-laws'.
People who use "I" in conversation before Their Majesties, how improper.
How certain low ranking officials live in their homes. At that rank they should live with their parents or parents-in-law, assuming there is room. Or borrowing one. Or something.
A man takes your letter and goes to read it in the garden. You chase after him, but you can not go past the curtains.
Hirsute, ugly men should not sleep by day, when people have to look at them.
She pities women who live out their entire lives in their husband's households, managing them, without ever knowing what the pleasures of court are.
A gentleman should not reveal his intimate relations with a lady by showing he is familiar with her name. He should slur it a little. At night, when looking for her, he should bring a servant to call for her, so his name will not be recognized.
People actually talk to their servants saying things, "As you kindly did for me." How shocking.
A dog went after a cat that had received a title of nobility; it was exiled, and when it came back, beaten savagely, to death they thought. When a dog appeared, they wondered whether it was the same, but it did not respond to its name -- which they deduced later was a way of hiding. (It was pardoned.)
A woman living alone should have a house that is dilapidated, and with weeds in the garden.