The changing voice of nonfiction

This one's for you, John.

A blogger I've just begun to read, Magic Molly, posted recently about a cookbook from 1927. I was struck by this quote:
The book is written in that kind omnipotent voice that was popular before we became self-obsessed and began to use personal pronouns like commas, around the late sixties (I think).
I suspect she's talking about a transition from passive voice to first- or third-person narrative, but I don't think often enough about the technical details of English usage to really be sure.

I enjoy trying to figure out what decade something was written by looking at the font and layout. It never occurred to me until now that perspective could change with the whim of fashion.