Understated, simple, close to characters' thoughts
Rainbow Rowell doesn't mess around in this book—she gets straight to the point. We don't have pages of flowery description here, or a lot of fancy language, and instead Rowell speaks like a teenager, letting her writing style reflect the youth and personality of her characters. She stays close to what her characters are thinking and feeling, and expresses it in a direct, often funny way.
Sometimes, Rowell even uses numbered lists to show Eleanor's thought process: "Eleanor considered her options […] 1. She could walk home from school […] 3. She could call her dad. Ha" (2.2-5). By using conversational language, humor, and slang, Rowell really nails the voices of her two protagonists.
For more on the style, be sure to check out the "Narrative Technique" section. The narration in this book is pretty unusual, and key to how the story is written.