What is the theme in Earth Abides by George R. Stewart?

Coming of age is an important theme in the book. Ish is a man who believes highly in intellect and places a great deal of trust in traditional knowledge and learning. In some ways, that makes him highly ill-equipped to deal with his current situation. However, he is able to look at things rationally and is a trained observer - both traits in his favor. It simply takes him a while to come to the point that he's able to live in his new world.