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.