"Posthumous" means written after death. The book is called The Posthumous Memoirs of Brás Cubas because Brás Cubas writes them already dead, and he explains the choice in the first chapter: "I am not properly a dead author, but a dead author, for whom the grave was another cradle." The phrase inverts the terms: he is not a writer who died, he is a dead man who writes, and the grave, for him, was a second cradle. Death is the place from which he speaks.
The dedication is already the dead man's: he dedicates the memoirs to "the worm that first gnawed the cold flesh of my corpse," as a fond memory. The book opens at the end, with the date of death, his age, his marital status, and the eleven friends who attended the burial, and only afterward goes back to his birth.
Being dead serves Brás to speak without fear. In chapter XXIV he says that "frankness is the first virtue of a dead man" and warns the living that "there is nothing so immeasurable as the disdain of the dead". Whoever risks nothing more can confess the interest behind every gesture, starting with his own: in the plaster that killed him, the main motive was seeing his own name in print, more than relieving humanity. The book leaves open whether this frankness is the freedom of one who risks nothing more, or the pose of one who chooses carefully what to show.