As Joker sits down and grabs one of the books, Strange can't help but scowl as he tries to grab said book away from Joker. He's not doing it even slightly delicately, another sign that he is already on the library's shitlist.

"And what do you think you're doing? Wait until I'm finished, then you can borrow the book."

The idea that Joker might want to help him research never even remotely enters Strange's mind. No, he must be doing this solely to be an annoyance.