Never had she appeared more serenely mistress of the situation than when, at the moment of dispersal, detaching herself a little from the group about the table, she turned with a smile and a graceful slant of the shoulders to receive her cloak from Dorset.
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.