When the Warden started booming, she had inconspicuously swallowed half a gramme of soma, with the result that she could now sit, serenely not listening, thinking of nothing at all, but with her large blue eyes fixed on the Warden’s face in an expression of rapt attention.
There are no more uses of "serene" in the book.
Show samples from other sources
While I lie awake worrying, she sleeps serenely.
She remained serene even when he was yelling at her.