The 2018 horror film The Nun does not adapt a named novel. Warner Bros.’ production description credits Gary Dauberman’s screenplay and the story by James Wan and Dauberman.
Why there is no matching book ending
The story involving Sister Irene, Father Burke and the Romanian abbey belongs to the film. Books about the Warrens, demonology or alleged paranormal cases are not therefore its original manuscript. They cannot supply a second version of the same ending for a book-versus-film comparison.
This page previously treated unspecified casebooks as the film’s literary source. That was misleading. There is no source novel to buy before watching this particular film.
Do you need to read anything first?
No. Follow the film franchise if you want its fictional continuity. If you want a horror novel with a religious or supernatural theme afterward, choose it as a separate recommendation rather than an official tie-in.
The books like The Exorcist guide is a useful reading route. Unlike a supposed novelization of this film, those recommendations are labeled by their reading appeal.
The official Nun II site also identifies the sequel as part of the Conjuring film universe and credits its screenwriters. That is franchise continuity, not evidence of an original Nun novel.