The library is definitely of interest to him; he can't think of a better way to pass the time in this dank, lonely place than by losing himself in books, in research and in stories, like a light in a dark place.

.. but the ultimatum is obvious there, in Dean's voice, and getting along and behaving himself through dinner may not be easy. Then again, Castiel has been raised in this sort of environment, bullied and harassed by too many older brothers with power gone to their heads, and he'd learned how to hold his tongue, how to keep his cool and retreat to a quiet place in his heart. He'll simply have to learn to do the same, here. So Castiel nods shallowly, and continues to follow.