Dry/Hallucinatory
D-503 starts the book writing in a very crisp, clinical writing style. He's not devoid of emotion, he doesn't really consider himself and individual and thus tries to speak the will of the One State:
I shall try to record only the things I see, the things I think, or, to be more exact, the things we think. (1.8)
As the story progresses and his humanity reawakens, however, he begins speaking in more poetic terms. At times, it becomes actively hallucinogenic and we're not quite sure if what he's relating is real or only a dream. That style suddenly snaps back in the final chapter, where he's had the Operation performed and again sees things only in stark mathematical terms. As with the tone, the writing style reflects a lot about the main character's state of mind, and the way it grows and changes throughout the narrative.