Quote 1
She talked of her own [childhood], too; but it was elusive, nameless, placeless, an impressionistic recital (7.2).
One of the ways Holly maintains her isolation is by only offering up generalities about herself. She appears to share some personal information, but it's too non-descript to really mean anything.
Quote 2
We wore the masks all the way home (7.6).
This is the scene when Holly and the narrator steal the Halloween masks from Woolworth's, and it carries a great deal of symbolic meaning in relation to the idea of isolation. There are a lot of masks Holly hides behind, and this allows her to maintain the isolation she seems to need so dearly.
Quote 3
I stepped on Holly's dark glasses, they were lying on the floor, the lenses already shattered, the frames cracked in half (11.8).
This occurs after Holly finds out about Fred's death, and the smashed glasses show us that she can't maintain total isolation from the people around her. What were once the symbol of her isolation – her glasses – have been destroyed when the narrator sees how profoundly impacted Holly is by the loss of her brother.