Fafhrd and the Grey Mouser by Fritz Leiber

If you're coming to these books for the first time, you'll be stunned at how much of the American fantasy tradition is founded on the salt flats, northern wastes, and strange seas of Newhon, the world where Leiber's fantasy is set. Fafhrd is a brawny, broadsword-wielding but surprisingly thoughtful northern barbarian. The Grey Mouser, his constant companion, is a sophisticated, cynical, nimble-fingered thief, and quite handy with a rapier. Their adventures are possibly the most satisfying blend of epic swordplay (the word "widdershins" comes up a lot, as in, "he spun his blade widdershins"), dark humor and arcane, pitch-dark magic ever penned. But the truly surprising thing about Leiber's Newhon books is their gorgeous, vibrant, literary prose: Leiber was no pulp hack, but a true artist of the word who kept one eye on his heroes' flashing blades, another on their sad, blackened hearts.

—Lev Grossman