So funny you chose that example as I was reading that line in the book on my iPad, which I have set to white on black background, because I find it less bright. The extensive white light on screens becomes hard to look at after a while. I’m with you to some extent with the grey on grey as long as the contrast ratio remains high.
I had wondered the same thing, so I decided to have a quick look around the web for some research on the topic. Turns out, it is damn difficult to find any sort of empirical test that consider BOTH readability AND eye strain. In terms of sheer readability, the research is pretty clear: Black on white always wins, and particularly so with participants with less than perfect vision.
At the same time, however, pure black on white puts a huge strain on our eyes later in the day, as bright white light pours from our monitors. It can be downright jarring to read anything after the sun has gone down, if the text is pure black on a pure white background.
It is also worthy of note that many dyslexic users are sensitive to brightness and very high contrast, which can cause words to blur together (according to uxmovement.com’s article, the source link, however, is broken.)
Well, there we go! Another good reason to soften up the contrast just a little bit.
Going into reading up on this, I actually thought that pure black on white would be optimal,
and that people just had to reduce the brightness on their screens.