The pages in books are a range of colours, very few are gloss white. I've just flicked through a few on my shelf; the whitest book I have is the wiring regs and they are notably less white than my wife's artists paper. Most of my books are a kind of murky brown not too dissimilar in mental feel to the HN background.

That’s why display brightness should be set such that it is similar to how a book page would look in comparable ambient lighting. “White” isn’t a specific brightness level.