One word: Skye. Thirteen Reasons Why ends with this one word, but boy is it packed with meaning. This word sums up the way that Hannah's tapes have affected Clay; and because the novel is centered on Clay listening to Hannah's recordings, their effect on him is a major issue.
Skye Miller is a girl that Clay used to crush on in middle school. After listening to Hannah's tapes, he recognizes that Skye is probably going through something, too; maybe something similar to Hannah's experience. Only after listening to Hannah's tapes does Clay realize that someone he used to care about has become isolated and withdrawn. He reaches out to her with one simple word – "Skye" (17.49) – and, in doing so, maybe saves a life.