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
presumes, without providing justification, that grammar books cannot have any use except as reference sources