Theo had kept his distance so as not to startle the stranger into brandishing a wand at him, a reaction that he wouldn't been surprised to face as it would likely have been his had he let his guard down for long enough to be startled. He gave the guy that same meaningless and polite little smile as he sat back and he had the books stacking themselves and neat with a quick movement of his 6" pine wand and some germanic sounding words.
He had the room to slot his own book back into the shelf, swapping it out for the book that was closest to him to read next when his attention was brought to the extra books. He was a friend of Seamus' then, maybe? "We could try a dimensional spell, make them all fit." he suggested, looking from the books to the shelves and back again. "That's a lot of new books. You know it's 'take one leave one', right?" There was a tone in his voice and a wryness to his expression that showed he was actually teasing.