A dim antagonism gathered force within him and darkened his mind as a cloud against her disloyalty and when it passed, cloud-like, leaving his mind serene and dutiful towards her again, he was made aware dimly and without regret of a first noiseless sundering of their lives.
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.