I can see why Neil Gaiman felt that Fritz Leiber deserved to have some of his work brought to the attention of 21st century readers in audio form. This book is a delight, a mix of classic swords-and-sorcery adventure, sardonic, dark fairytale, and imaginative world creation, with a little tales-of-ribaldry kinkyness thrown in. While it's fifth in a series, I don't see any reason you can't start here. The hairy barbarian Fafhrd and the small, quick-witted Gray Mouser are two instantly familiar roguish heroes, no introduction required beyond the first chapter, and Lieber quickly pulled me into their world with his deliciously visual, textured descriptions and playful, literate command of language. Fans of Jack Vance will find his style familiar, though it's less absurdist.
The story here has Fafhrd and the Gray Mouser low on cash, and compelled to take a commission guarding a grain barge for the amusingly decadent ruler of the impressive, seedy city of Lankhmar. Once out to sea, they learn that their convoy is also carrying a not particularly innocent maiden and her collection of preternaturally intelligent rats. Soon, things go amiss, and our heroes find themselves headed, by separate routes, back to Lankhmar, which is now having some serious rat problems. Any not just any rats, but ones that seem to be more and more humanlike, and to be coming from somewhere under the city. I won't spoil what happens next, but before all is said and done, there will be duels, ill-advised romances, spying in magical disguise, battles, grotesque sorcerers, strange creatures, otherworldly travelers, and a few mildly naughty scenes.
IMO, this is fantasy that's a happy medium between the grimness of Howard / ponderousness of Tolkien and the silliness of Vance, pulpy but actually creative. It's not hard to to see the influence Leiber had on more modern writers in the genre, from Terry Pratchett to David Eddings to China Mieville (particularly the weird romance) to Neil Gaiman himself. Audiobook narrator Jonathan Davis does a fine job as usual, his calm, arch style a great fit for Leiber's writing (though his scene switches are a little abrupt).