There are 3 books in the Ashfall Trilogy series. The newest book is Sunrise which came out in 2014.

1
Ashfall

Book cover of Ashfall
Many visitors to Yellowstone National Park don't realize that the boiling hot springs and spraying geysers are caused by an underlying supervolcano, so large that the caldera can only be seen by plane or satellite. And by some scientific measurements, it could be overdue for an eruption. For Alex, being…
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2
Ashen Winter

Book cover of Ashen Winter
It's been over six months since the eruption of the Yellowstone supervolcano. Alex and Darla have been staying with Alex's relatives, trying to cope with the new reality of the primitive world so vividly portrayed in Ashfall, the first book in this series. It's also been six months of waiting…

3
Sunrise

Book cover of Sunrise
The Yellowstone supervolcano nearly wiped out the human race. Now, almost a year after the eruption, the survivors seem determined to finish the job. Communities wage war on each other, gangs of cannibals roam the countryside, and what little government survived the eruption has collapsed completely. The ham radio has…