The sound of this line, with its repeated s sound (a.k.a. "sibilance"), reflects the meaning of the line as well: the line sounds as soothing and "serene" as her thoughts supposedly are.
How pure, how dear their dwelling place. (line 12)
The speaker assumes that the calm and "serene" expression on the woman's face reflects the "pur[ity]" of her mind.
But tell of days in goodness spent, (line 16)
The speaker also assumes that the woman's sweet expression "tell[s]" the story of her past goodness – that's a lot to read just from someone's facial expression.
A mind at peace with all below, (line 17)
The calmness of her face makes the speaker assume that she's "at peace" with everyone. There's nothing to make her anxious or worried.
A heart whose love is innocent! (line 18)
In the final line of the poem, the speaker reassures us (in case we were suspicious) that the woman is "innocent." She's not going to allow him to seduce her, or do anything else of that nature, so stop thinking that.