The american Poul (William) Anderson (1926-2001) was one of those authors who both wrote fantasy and SF. His first piece of fantasy was The Broken Sword (1954) which based on much things out of the scandinavian mythology. The story is dark and the mortal ones are been used by the gods, who are afraid that the mortals shall see the real power they have, and destroy the gods.
Three Hearts and Three Lions (1953) is a somewhat lighter story about Holger who reluctantly becomes a hero when he is transefered to a darker world and there ends up in the battle between the gods. A Midsummer Tempest was the name of the sequel which appeared 1974.
Poul Andersons most famous tale of fantasy: "The King of Ys" did he write together with his wife Karen Anderson. The story consists of the novels Roma Mater (1986), Gallicenae (1987), Dahut (1988) and The Dog and the Wolf (1988). The books are based on a legend about the town Ys, and tells a story about the Romans and their fight with the free northern countries during the 5th century.

- Ursula Le Guin

Books at Amazon.com (USA):
Three Hearts and Three Lions
The King of Ys