Matter of Fact, Concise
No frills or fancy word-stuff as far as the eye can see. This novel is written in a direct and to-the-point manner, using basic language to directly tell the reader what is going on in the narrative. Sure, we know what Asher's feeling, but that's only because he outlines it for us directly, like in this paragraph about how little he wants to leave Crown Heights for Vienna:
I lay in my bed with my eyes closed and did not want to leave my room. I thought at one time during that week that a violent snowstorm raged outside; but I was not sure and I did not care. I did not want to leave my room. I did not want to leave my street. (93.2)
A good explanation for the simplistic language may be the fact that Asher is writing from a kid's point of view, and kids aren't exactly great pontificators. The major benefit of the matter of fact and concise style is that the story is very easy to understand, which means we as readers can spend our time dealing with its poignant emotions instead of looking up words in the dictionary:
I would not paint on Shabbos. I spent Shabbos mornings praying and reviewing the Torah reading. I spent Shabbos afternoons studying a book on Hasidus I had brought with me. Jacob Kahn spent Shabbos mornings on the beach with his wife and Shabbos afternoons painting. (258.3)
The cultural gap between Asher, a religious Jew, and Jacob Kahn, a non-practicing Jew, is about fifty miles wide here. And because it's laid out in Asher's simple and direct language, it is far easier to grasp that it would have been if it had been written in flowery purple prose.