Swords and Deviltry (1970) by Fritz Leiber were not the first Fafhrd and Gray Mouser stories ever written, but there origin stories here give us a place to begin. One part tall tale, one part comedy, one part your Uncle's war story, and three parts literary trainwreck, this book gives us three grindingly dull tails of this thieving duo.
While I enjoyed the comedy and light-heartedness of these stories, often dipping their feet into absurdism, these delights were enough to save me from plodding storytelling, where every move of the main characters, even the dull and uninteresting moves, were described to us.
As one of the progenitors of D&D, this story is filled with tropes that would show up in D&D, such as flaming oil, the spell Guards and Wards, a Thieve's Guild, thieves using slings, characters with no moral reasoning whatsoever, slinking through passages and exploring for each room, and, of course, sorcerers. If you are ever interested in the early forms of D&D, the last story especially reads like an actual D&D game from the era.
Unless you have a specific technical or historical reason to read this book, don't. Books like this are the reason that fantasy has a bad rap for being terrible literature.