That was an interesting reaction. William took it to mean that Nate fervently wanted no further assistance in his current studies, and opted to let the matter drop. He did make a noncommittal noise, accompanied by a sideways glance, but didn't question any further. Nate would tell him if he needed William's help, and even if he didn't, it wasn't as if Nate could check books out of the library without William knowing what they were. He could always indulge in a little investigation.
"I'm here if you need me," William told him, casting a quick dusting-up spell on the vacant spaces where Nate's selection of books had recently been and snapping the remaining volumes to tidy attention.
It was possible he lingered slightly over one of the carved wooden bookends, polishing it with the sleeve of his robe, before asking quite casually, "You're still assisting Professor Saporta in his advanced classes, aren't you? Would you mind asking him a question for me, when next you see him? It's nothing critical," he assured Nate quickly, attention on the bookend. "But I would appreciate it, if it's not inconvenient."