A pillow book is also the name given in the West to collections of erotic Japanese woodblock prints (shunga) dating from the 17th - 19th century depicting various sexual positions and practises ranging from the mundane to the athletic to the utterly impossible.
Expressions are lifelike: very few of these people are smiling. Instead they have the pained/angry expressions of two people getting it ON! as opposed to the pasted-on grins of Western erotica, with the male looking decidedly determined, and the female's head often tipped back in an orgasmic sob, her back arched in mammalian lordosis. Genitalia are often drawn with clinical precision, but not always to scale: often the penises are HUGE fuckers (athem), advancing on the hapless vulva with the ferocity of Musashi dispatching a bandit. To add to the general swing of things, often both partners are usually wearing kimono (not a bad idea, considering, since often I've felt rather cold about the shoulders whilst in the act), adding to the sinuous lines of the intertwined bodies with decorative swirls of clothing, moreover, they often sport elaborate hairstyles as well (not unsurprisingly, styles associated with high-end prostitution --tayuu, or oiran-- abound, though the more modest "geisha" or even "housewife" styles are seen, now and then).
Western lore about pillow books, as with a lot of other Asian erotic art, is extensive, romantic, and mostly false: at various times, pillow books have been thought to be marriage manuals, given to newlyweds to avoid the embarrassment of the traditional mother-to-daughter talk ("Remember, it happened to the Queen.") common in Western cultures, checklists for sophisticated lovers among the aristocracy, even religious art. Most scholars today are agreed that their intent and use was pretty much the same as pr0n nowadays: cheap stroke material available in the red-light district as a souvenir or sex-to-go, so to speak, along with portraits of the reigning beauties of the day, pictures of actors, views of mountains (with cute chix0rs in foreground) etc. Wildly popular, shunga account for up to 50% of all woodblock prints made, despite occasional censorship.