The answer is 1/2. This is because the question says that 2/3 of those are historical. By those, the question refers to the fraction of books that are fiction. In mathematical questions, 'of' usually refers to multiplication. Since the question says' 2/3 of those'. It means 2/3 multiplied by 3/4. And this gives 1/2 or 0.5. Hope i helped.