John Hill’s book “A Guide to Contemporary New York City Architecture” is filled with examples of the crazy new forms of the last decade, like Frank Gehry’s white wind-filled “sail” on the West Side Highway in Chelsea. They are startling, creative, amusing, sometimes even hilarious.

And yet, the United States is in the middle of a great revival of traditional architecture — Georgian, neo-Classical, Arts and Crafts and so forth — that is almost absent from Mr. Hill’s stimulating and enjoyable work. So, what isn’t contemporary about traditional design?

“Modern” didn’t used to be such a big deal — it was just what was lying around at the moment. Indoor plumbing, Queen Anne, mansard roof, Romanesque, Corinthian columns — they all had their 15 minutes of modern, and then, without much fuss, they were succeeded by something different.

But in the 1930s the definition of “modern” became hardened into antihistory: glass boxes with flat roofs, or at least nothing observably traditional. Modern was morally correct; anything else was trivial, or worse.