This question already has an answer here:
- “How deep” or “How deeply”? 2 answers
This is from Light in August, by W. Faulkner, Chapter 18:
He began to breathe deep. He could feel himself breathing deep, (...)
This confused me deep.
(This is related but not an answer to my question. Read especially the comments to the accepted answer.)
EDIT. As noted in the comments and in the title: the question is indeed why deep and not deeply? Even if it does not feel too wrong, the second statement certainly confuses readers deep. Even if the use of deep as an adverb is widely accepted in formal speech, one does not expect to hear from the narrator in a novel. [This is an adverb, right? Because like I said, I am confused.]
For rerefence, the rest of the sentence is:
He began to breathe deep. He could feel himself breathing deep, as if each time his insides were afraid that next breath they would not be able to give far enough and that something terrible would happen, and that all the time he could look down at himself breathing, at his chest, and see no movement at all, like when dynamite first begins, gathers itself for the now Now NOW, the shape of the outside of the stick does not change; that the people who passed and looked at him could see no change: a small man you would not look at twice, that you would never believe he had done what he had done and felt what he had felt, who had believed that out there at the mill on a Saturday afternoon, alone, the chance to be hurt could not have found him.
The full text can be found here, page 168.