Poul Anderson 
Winter of the World [EPUB ebook] 

The second Ice Age came – and civilisation was buried beneath the glaciers. Then all men became barbarians. But out of the frozen chaos three cultures emerged. The Seafolk and the Rogaviki were simple, primitive people. Ranged against them was the mighty Rahidian-Barommian Empire, a nation hungry for power, eager to exploit its superior technical knowledge. If the Empire fulfilled its ambitions, its neighbours knew they would not survive. On the day of the intended conquest they must be ready to engage the powers of science in a bloody, apocalyptic struggle for the destiny of the planet.