California by Edan Lepucki
In the near future society in the United States has crumbled into a series of walled communities who primary commodity is safety. Cal and Frida don't live in a community but in an isolated rural area. When Frida discovers she is pregnant she starts to desire other company than just Cal. So they set out to the nearest settlement. It seems things will be better in the company of others but secrets begin to emerge that are disturbing to the couple.
You don't really get too much of why the United States has failed except for mention of a series of calamities. This is more a quiet novel about the nature of man when the thin veneer of civilization cracks.