The AHD has long been on my list of big, expensive books I really want, and your notice has my mouth watering.

I like Nunberg a lot--he's one of the best reasons to listen to Fresh Air--and his approach to usage is very nicely stated. It reminds me a bit of Sir Ernest Gower's remarks on what he called "the proper use of words":

Professional writers realise that they cannot hope to affect their readers as they wish without care and practice in the proper use of words. The need for the official to take pains is even greater, for if what the professional writer has written is wearisome and obscure the reader can toss the book aside and read no more, but only at his peril can he so treat what the official has tried to tell him. By proper use I do not mean grammatically proper. It is true that there are rules of grammar and syntax, just as in music there are rules of harmony and counterpoint. But one can no more write good English than one can compose good music, merely by keeping the rules. On the whole they are aids to writing sensibly, for they are in the main no more than the distillation of successful experiments made by writers of English through the centuries in how best to handle words so as to make the writer's meaning plain.