1968's Swords of Lankhmar marks the first Fafhrd and Gray Mouser novel. In this single story, Lankhmar is threatened by intelligent rats.
As Leiber novels go, this one is almost coherent, most likely due to the novel having an editor. God bless that unknown editor, who with a baseball bat and chainsaw, somehow almost got Leiber to produce a coherent novel. For the most part, the events unfold in an orderly and comprehensible manner, story arcs are begun and completed well, and side narratives are kept to a minimum. On the downside, about half the novel is filler and could easily be cut, taking this work down to novella length with nary a blink. In some areas he still tells a story back-assward, but these episodes are now in the minority.
As a work of fantasy comedy, the whole thing works astonishingly well. Plant your tongue in your cheek and chuckle from beginning to end. Where this book works, it works quite well (assuming that the prose doesn't undermine it). The absurdity of rats conquering Lankhmar in the first step to conquering the universe is the pinnacle of anti-heroicism.
Even with praise, I can't generally recommend the book. If you like fantasy humor and you know the old genre, this book is a good choice for you. Outside of that, I'm very iffy recommending it at all.