I think you are talking about a place in the book where Guy Montag has just been talking to Clarisse.  She has asked him if he is happy.  He comes home, opens the door, and pauses in the hallway looking up at the ventilation grille (where his books are).  While there, he contemplates a couple of things.

First, he thinks very briefly about having met Faber in the park once.  Then, he starts thinking a lot about Clarisse and what she means to him.  Most importantly, he thinks about how she reflects his own innermost thoughts back to him.