How is the theme in The Story of Edgar Sawtelle developed throughout the novel?

One theme that cannot be ignored in this lengthy novel is
the idea of isolation, shown most prominently in the character of the protagonist,
Edgar.  First, he is mute, but not Deaf.  His isolation is defined by his ability to
hear and fully understand the dialogue of those around him, but not necessarily to
express himself in return.  As a result, he is closest to his own parents, with whom he
shares a sign language that he has largely created, and the
dogs.

This theme is further developed when Edgar's father
dies.  As the only person around at the time to save him, Edgar cannot shake the fact
that his father's death was in large part, his fault, because he cannot call someone on
the phone for help.  His isolation increases with his inability to express this fact to
his mother and others.

Then, his father's ghost appears to
Edgar one night in the barn, and reveals a secret so deep (and so personal) that Edgar
isn't even sure if the experience actually took place.  He burns with the assumption of
knowledge that his own uncle killed his father, but retreats inward with this secret
because he does not know what to do with it.

Finally, when
Edgar accidentally causes Dr. Papineau's death, he runs away and ends up living in the
forest with three of his dogs.  Though he is able to communicate and mostly understand
the dogs (and they, him), this is the peak of his isolation.  The physical isolation of
his hiding and often going days without seeing nor hearing anything other than the
sounds of nature seems to match his internal and psychological
isolation.