Mark Haddon's narrative pulls along at a very leisurely pace, his words are pithy and laconic, e.g., "He flushed the toilet and washed his hands. Bed." Another innovative use of language is in the dialogue which also display a tendency towards taciturnity. Short. Telegraphic. Modern texting teenagers would approve.
When style is visibly displayed it becomes a distraction, so the narrative ambles along at its own pace. However, a good read on a rainy day, when you have nothing else besides hot coffee.