Dangerosa’s suggestion of “books about science/social sciences/current events” seems pretty good to me.
My check-off on some recent suggestions: No on Fast Food Nation, Into Thin Air, Devil in the White City (though this latter is on the mental list of “if you see it in the library or on a remainder table, pick it up” books), The Elegant Universe, and Goedel, Escher, Bach.
I own both Joy of Sex and Our Bodies Ourselves, but both are more like, ahem, reference books, in that it’s not about sitting down and reading them straight through.
Oh, and I forgot to mention before – I’m not sure I want to consider Heartbreaking Work… as nonfiction in the more restrictive sense we seem to be moving towards. Memoir, yeah, but …