I'm in the middle of reading Mike Mullin's Ashfall.
It's a real page-turner, like another book I read earlier this year with a similar title (Ashes by Ilsa Bick). In Ashfall, Alex is 15 when his parents and little sister go out of town for the weekend, leaving him alone - and the super volcano in Yellowstone blows. Now Alex is left in a world where you can't even see the sun, huge drifts of ash collapse roofs and cover fields and roads, and people are desperate for food and water. It's a post apocalyptic novel that could really happen. Be warned - there is a fair amount of violence that Alex witnesses or even commits (but only when necessary).
In an interview with School Library Journal, Mullin reveals that the manuscript was rejected at some stage by 24 literary agents. Read more here.