Walt Whitman: INFP
P because "I have instant conductors all over me, whether I pass or stop;
They seize every object and lead it harmlessly through me." seems to me thee essence of P-hood, I because although he purported to be all open and everywhere, he had to have Ni in order to know so much about himself (and the subject of all of his poetry is either himself or himself relative to Abraham Lincoln, right?) N just seems obvious, and F because I don't think that any T in history could put up with "do I contradict myself? Very well then I contradict myself." Plus the parts of "When Lilacs Last..." that I've read seem very emotional.
I'd consider Shakespeare an ENFP, because it seems to me that he must have had Ne because all of his work is about understanding other people well enough to create characters who seem like real people. Plus, he was an actor and acting seems to be a very ENFP friendly type profession. And, unlike Whitman, he subject seems to never be himself. All in all, based on what I know about his biography, he seems to be an ENFP who had to take on some more SJ qualities now and then as a self-preservational instinct. Plus, the whole "fall in love with Anne whatshername and then get bored with her and run off to London without her" thing seems ENFP-like: all interested in possibilities and horribly bored when they seem to be exhausted. But I probably just want to claim Shakespeare for my own type; who wouldn't want him?
Also, on that note, and on the basis of a very little amount of reading, I'd consider D.H. Lawrence rather ENFP as well, because he talks about how he wants his wife to be constantly changing even as he is constantly changing. Sounds like possibility-lust to me!
And I don't know if we're counting critics, but since it just says "authors" I think that the critic Harold Bloom is definitely an NFJ but I can't tell if he's I or E. Also, there's a slim possibility that he might be a P who looks like a J because he's so confident in his intuitions, but I doubt it. My guess is INFJ.
Based on Miller's work, I'm inclined to call him INF, but I'm uncertain whether the heavy judgment in many of his plays (from "attention must be paid" to the irritating and irrelevant moralizing in the lengthy stage direction thingys in The Crucible) stems from an actual judging nature or just deeply held beliefs being expressed strongly.