The Inferno by Dante Alighieri. A book that was described by Harold Bloom in the following way:
Nothing else in Western literature, in the long span from the Yahwist and Homer through Joyce and Beckett, is as sublimely outrageous as Dante's exaltation of Beatrice, sublimated from being an image of desire to angelic status, in which role she becomes a crucial element in the church's hierarchy of salvation.
In Bloom's description you can see why the book is both important and controversial. Though to be fair, Bloom is talking about the entire Comedy where the book-banning crowd want to single out Inferno.