The Last Wish by Andrzej Sapkowski

Geralt the Witcher—revered and hated—is 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 murderer: his targets are the multifarious monsters and vile fiends that ravage the land and attack the innocent.

But not everything monstrous-looking is evil and not everything fair is good... and in every fairy tale there is a grain of truth.