The author explicitly says they're talking about this genre. From TFA: "By 'non-fiction' I mean mass-market non-fiction, those paperbacks with titles like 'Sle

The author explicitly says they're talking about this genre. From TFA: "By 'non-fiction' I mean mass-market non-fiction, those paperbacks with titles like 'Sleep: Why We Need It, And Why We Don’t Get Enough of It'. Textbooks and technical books are a separate category." I see at least three categories - mass-market, textbooks/technical, and your "higher echelon".