Novelist: Any author who thinks a sentence is ungrammatical will not write it down in the first place, and thus will have no need to use a grammar book  On the other hand, any author who is sure a sentence she or he has written is grammatical will not feel a need to consult a grammar book
Thus, grammar books are useless as reference sources for authors
infers, from the claim that authors should not consult grammar books, that they will not in fact do so