The Last Wish by Andrzej Sapkowski

Geralt is a Witcher, a man whose magic powers, enhanced by long training and a mysterious elixir, have made him a brilliant fighter and a merciless assassin. Yet he is no ordinary killer. His sole purpose: to destroy the monsters that plague the world. But not everything monstrous-looking is evil and not everything fair is good…and in every fairy tale there is a grain of truth.