as far as I am aware, you can bring your books in whatever you want to bring them in (many in my state carried backpacks). That doesn't mean the books stay in that vessel for the exam. you will likely bring your books up to the table or adjacent on the floor for the actual test. The only weird rule they gave us was we couldn't roll our suitcases on the escalator. we had to carry them instead. which, if you have any logic in your brain, that puts even more weight on one step but WHO AM I TO SAY what makes sense. lol.
basically, bring whatever is most comfortable for you.