if P=NP, could a computer write essays and novels?

No. If there existed a polynomial time algorithm for determining if a novel was good, then with P=NP a computer could in a sense try all possible sequences of words and determine if any of them was a good novel, and output that one. But there does not exist any known algorithm, polynomial or otherwise, for determining if a novel is good.

Of course if you don't require that it be good, I think a computer could write a novel today.