J. J. M. Roberts describes the unity in the book of Isaiah.
J. J. M. Roberts, "Introduction to Isaiah," HarperCollins Study Bible, ed. Harold Atrridge (San Fransisco: HarperCollins Publishers, 2006), 914
An Overarching Unity
Despite the different historical settings to which the various parts of the book may be attributed and the clear evidence of multiple authorship, there is also a certain overarching unity to the book. All parts of the book share a common theological tradition, and the later parts seem to take up, adapt, respond to, or reverse announcements made in earlier parts of the book while staying in that same theological tradition. Like most other prophetic books, the book of Isaiah moves in a general way from oracles predominantly of judgment to those of salvation. In response to these observations, recent scholarship has begun to turn its primary attention away from the traditional historical-critical issues of historical setting and authorship toward an attempt to explain this overarching unity. Some scholars see this unity as the result of a thorough reediting of the book by the latest authors or their disciples in the exilic or postexilic periods. In its most extreme form, this scholarly trend also explores how the prophetic collection as a whole has been edited to shape interconnections with the whole prophetic corpus.
Most scholars, even of the most traditional historical-critical mold, would admit there has been some late editing of the earlier parts of the book, but the evidence for a thorough, intentional, coherent editing of the book as a whole is not very persuasive. Such reconstructions are far more hypothetical than the reconstructions of traditional historical criticism. Historical reconstructions at least have attested external historical events as a control over pure speculation. Battles and political crises are public events that leave records elsewhere than in the prophetic book itself. In contrast, editorial work by its very nature is private and, barring the discovery of earlier manuscripts showing variant text forms, the reconstruction of such work never rises above the level of more or less plausible hypotheses. In this author's opinion, the editorial processes even within the material that may be confidently attributed to the eighth-century BCE Isaiah of Jerusalem are often less than transparent. Thus the opinion reflected in these notes is that the overarching unity of the book owes more to a common theological tradition in which all the authors stood than to any consistent and coherent editing the book has undergone. The common theological tradition and the fact that the later authors were responding to and commenting on the earlier oracles in the book are sufficient explanation for the overarching unity.