"What are you reading?" "A book"

From same source:


A: What are you reading?
B: A book.

"This raises an implicature. B reasons (unconsciously) as follows"

"I asked A what he was reading."

"My question required him to tell me either the title of the book or at least its subject matter."

"Instead, he told me what I could already see for myself, to wit: that he was reading a book."

"He appears to be flouting the Maxim of Quantity."

"There must be a reason he gave less information that the situation requires."

"The inference that I draw is that he does not want to be disturbed."

"Also, that he is trying to end the conversation."