"These novels will give way, by and by, to diaries or autobiographies—captivating books, if only a man knew how to choose among what he calls his experiences that which is really his experience, and how to record truth truly." Ralph Waldo Emerson
This epigraph is basically Miller's way of saying, hey, look what this great 19th-century American essayist thought of romanticized autobiography. It's the best thing out there! Better than novels!
Now keep that in mind as you read Miller's proclamation at the beginning of chapter one:
This is not a book. This is libel, slander, defamation of character. This is not a book, in the ordinary sense of the word. No, this is a prolonged insult, a gob of spit in the face of Art (1.6)
With Emerson's support, Miller sees that books as we know them have to change—and will change—because of authors like him who write straight from the gut.