Poul Anderson 
Valor of Cappen Varra [EPUB ebook] 

We have said that there are many and strange shadows, memories surviving from dim pasts, in this FANTASTIC UNIVERSE of ours. Poul Anderson turns to a legend from the Northern countries, countries where even today the pagan past seems only like yesterday, and tells the story of Cappen Varra, who came to Norren a long, long time ago. Let little Cappen go, they shouted. Maybe he can sing the trolls to sleep