For novices, Lloyd Hopkins trilogy (Blood Moon, etc.) or his first novel (Brown's Requiem) are more appropriate to discover the author and her particular universe: very black, very raw and simultaneously a beauty poignant, deep humanism because without concessions.
The Black Dahlia is all this at once, embedded in an abundant historical panorama that revives the LA 50's with an almost palpable touch of reality.
So of course it sometimes takes long and lovers of thrillers general public will be a little confused by the lack of narrative structure "classic" with mandatory twists, and any personal involvement is felt Ellroy in this story that it is appropriate.
The Black Dahlia should not read like a thriller, but as the story of a quest, physical and spiritual, on the path of truth and redemption in the midst of a dark and violent universe.
From this perspective, the novel then reveals all its beauty, all its darkness and sheds its narrow gangue "polar" to show itself as one of the great contemporary American novels. Simply.