Test of Fire

A small group of survivors fight to rebuild civilization after the Earth is devastated by a huge solar flare.
Cities became ovens. Grasslands became seas of flame. As the touch of dawn swept westward across the spinning planet Earth, its fiery finger killed everything in its path. Glaciers in Switzerland began to melt, floodwaters poured down on the burning, smoking villages dotting the Alpine meadows. Paris became a torch, then London. North of the Arctic Circle, Lapplanders in their summer furs burst into flame as their reindeer collapsed and roasted on the smoking tundra.
The line of dawn raced westward across the Atlantic Ocean, but as it did the brightness diminished. The sun dimmed as quickly as it had brightened.
The Americas escaped the Sun's wrath. Almost.
"A hard, dark book, the story of mankind after the fall... compulsive reading... the battle to rebuild Earth after its almost total destruction by a gigantic solar flare."
—Harry Harrison