While I don't endorse every comment made in this article, the following remarks are worth posting in light of past discussions we've had about Isaiah's place in the canon:
The final stages of the formation of Isaiah, including the appearance of something like its present form, occurred sometime between the 5th and 3rd centuries bce. More precise dates remain contested, owing to the paucity of both specific historical references in the later chapters of the book and external historical sources from this period. The Great Isaiah Scroll from Qumran (1QIsaa)—a complete copy of the book whose content is very similar to manuscripts from nearly a millennium later—dates to the late 2nd century bce; near the beginning of the same century, the author of the biblical book of Sirach knew a form of Isaiah that included both chapters 36–39 and 40–55 (Sir. 48:23–24). Additional editorial work may have continued until the turn of the Common Era, as suggested by differences among copies of Isaiah from Qumran, the early Greek translation of Isaiah in the Septuagint, and later Hebrew manuscripts.9 At the same time, Qumran marks the point at which interpretation of Isaiah began to appear in separate commentaries on the text, instead of additions to the book itself.10 Although it would be historically anachronistic to speak of a biblical canon at this point, this development suggests that Isaiah was viewed as possessing some degree of religious authority.