Now that I have had the entire night to allow my feelings to relax, I will offer a pillow book summary while trying to keep as many spoilers out as possible. Essentially, the novel follows FJ and DH, main characters in eternal love who we all know don't wind up together. DH insists that he and FJ have no destiny, no fate. This is a central premise of Pillow Book. It asks the question, can love overcome the worse odds, even fate?
DH (unlike in the show) encounters FJ for the first time in the beginning of the novel (or he at least thinks this is their first meeting). He never once noticed her as a maid and he never knew the fox he was taking care of (apparently he has a soft spot for animals, and FJ is described to be an 'especially alluring fox' because she is red, and most often disguise this) is FJ. (In the show he is shown to have many interactions with her as a maid and def. who the fox he was caring for was). A number of misunderstandings arise, almost entirely because of FJ's youth (she isn't the most experienced and is naïve, often overestimates her abilities, including smarts) and DH's indifference (if only he was a less sadistic man).
After a series of extremely shameful and embarrassing encounters with DH (of which he deliberately tries to make worse in order to watch FJ's extremely upset reactions--he is, as I said, a bit sadistic because he is bored and looking for some entertainment) she winds up sealed away in a valley (partly DH's fault but he is never going to acknowledge that). We learn a great deal about FJ's one sided pursuit of DH through a series of flashbacks. I genuinely pity her. Because the worse thing is DH never even cares to notice her--like she is so beneath his notice.
After a series of events, which I will not explain, DH eventually falls in love with FJ but his way of handling it is rather ruthless. Like once he sees something he wants he will go to great lengths to secure it. I'm not saying he does anything so shameless as to physically hurt her but he uses his smarts to heavily manipulate many situations to his advantage. He does this because he is trying to go against fate. Whether he succeeds...well that is something the novel will tell you about.
In addition, the second part of the pillow book does something similar to eternal love: just as BQ was SU Su and Si Yin and A-Yin, our FJ is the fox, then she is just regular FJ, then she is Arayna. DH becomes Xize. As Arayna and Xize we see their relationship change a great deal, and for the first time DH must contend with extremely alien feelings to him: lust, love, guilt, and jealousy. Essentially everything he is not supposed to feel. No, he is not human. He is posing as an immortal. We also learn more about his past and how he was raised, etc.
That is all I am going to say about the novel because if I say anything more I would be giving away a lot of spoilers. I tried to convey the bare bones of the novel without sharing any shocking plot twists. Miscommunication is also one of the driving plot devices in this novel, arguably the result of FJ taking things at face value (remember, not the brightest bulb, but she has a lot of heart), and DH not really caring enough to say anything or even notice that someone might misinterpret something (is the polar opposite of FJ in just about every way. so unlikely of a couple that at one point people place bets to see who DH will wind up with and no one bets on her.)
