"I'd investigate." Which feels self-evident, but Yellow... Arthur never taught him that, not the way he did John, and so he's happy to spell out for him, so he can make that choice for himself. "I'd... try and find how if there's any information about the Necronomicon other people know. Who might be looking for it, if they hear that the pages been stolen or used, what sort of things have happened to other people that might have read it."