I think most people will agree that Ayn Rand was an INTJ. However, if you consider only her writing style, she was an ISTJ.

Examples of Rand's ISTJ writing:

"A circle, she thought, is the movement proper to physical nature, they say that there's nothing but circular motion in the inanimate universe around us, but the straight line is the badge of man, the straight line of a geometrical abstraction that makes roads, rails and bridges, the straight line that cuts the curving aimlessness of nature by a purposeful motion from a start to an end."

"The great oak tree had stood on a hill over the Hudson, in a lonely spot on the Taggart estate. Eddie Willers, aged seven, liked to come and look at that tree. It had stood there for hundreds of years, and he thought it would always stand there. Its roots clutched the hill like a fist with fingers sunk into the soil, and he thought that if a giant were to seize it by the top, he would not be able to uproot it, but would swing the hill and the whole of the earth with it, like a ball at the end of a string. He felt safe in the oak tree's presence; it was a thing that nothing could change or threaten; it was his greatest symbol of strength."

"The symbol of all relationships among such men, the moral symbol of respect for human beings, is the trader. We, who live by values, not by loot, are traders, both in matter and in spirit. A trader is a man who earns what he gets and does not give or take the undeserved. A trader does not ask to be paid for his failures, nor does he ask to be loved for his flaws. A trader does not squander his body as fodder or his soul as alms. Just as he does not give his work except in trade for material values, so he does not give the values of his spirit—his love, his friendship, his esteem—except in payment and in trade for human virtues, in payment for his own selfish pleasure, which he receives from men he can respect. The mystic parasites who have, throughout the ages, reviled the traders and held them in contempt, while honoring the beggars and the looters, have known the secret motive of their sneers: a trader is the entity they dread—a man of justice."