Requiem for a Nun is the sequel to Faulkner's early novel Sanctuary, and has the same main characters i.e. Temple Drake, Gowan Stevens, Gavin Stevens. The events in Requiem are set in Faulkner's fictional Yoknapatawpha County and Jackson, Mississippi, in November 1937 and March 1938, eight years after the events of Sanctuary. Temple and Gowan are now married with two children (one, however, is dead) and Temple's violent past comes back to haunt her. The nun of the title is Temple's nanny, an ex prostitute that she met whilst working as a prostitute, accused and convicted of killing her baby and is about to be hung. Apparently Elizabethan era-slang for a prostitute is nun and its contemporary meaning is of a woman who sacrifices herself to save sinners, which is what happens in the novel.
Faulkner's text is - of course - suitably challenging. The book is part prose and part play. The chapters being interspersed. The prose chapters consist of unending sentences but are broken into sections, which is interesting, and these chapters tell the history of Faulkner's fictional Yoknapatawpha County and Jackson, Mississippi. Each prose chapter details the history of a particular institution, like the jail, in which the play like dialogue takes place.
As always, Faulkner is superb and the book is thoroughly fascinating. Although the reader has to work hard to understand what he's written it's well worth the struggle. This is his most obscure book yet.