For those who wish to get right to the heart of the matter, here is my conclusion.
The passages by some referred to as the Deutero-Isaiah are a source of much debate historically, semantically, and theologically. Gerhard von Rad believes there was not only a second author, but also a third author followed up with some redaction. Various scholars hold this point. Brueggemann, Barr, and Waltke among others have a problem with the shift of both style and content between the First Half (1-39) and the Second Half (40-66). But the discovery of the Great Isaiah Scroll found in Qumran in 1947 and with the textual criticism work between LXX, DSS-IQ, and MT, there seems to be a growing effort to put emphasis on Isaiah as a single work by a single author.
This author has sought to show Isaiah is a single document from a single source. The history of the debate over Isaiah detailed the position of many church scholars, to include early efforts, Middle Age, Reformation, and post-modern positions. In addition, a detailed description of hermeneutical methodologies necessary to understand the original intent of the author here given including, differences in societies, principles discussed, and applications.
Walter C. Kaiser Jr. has done extensive study into biblical theology of the Old Testament. As pointed out earlier, Kaiser bases his biblical theology on promises made by God to Israel. Kaiser sees three main promises: (1) Abraham would have a great family and through him bless all the world, (2) God promised Moses his blessings if the nation of Israel would obey his commandments, and (3) the promise to David that his descendants would forever sit on the throne ruling Israel. Kaiser saw this promise theology detailed in the prophecies of Isaiah.
Within the Second Half of Isaiah, the passages are in three enneads. The first speaks about God the Father. Three points were detailed concerning God the Father: (1) God is incomparable, for he is unique and no one is like him or compared to him, (2) God is the creator of all, and (3) apart from God there is no other. The second speaks to the subject of the Servant, seen as one coming with the blessings of God. The servant will suffer for many by a substitionary sacrifice of himself. The pinnacle of this ennead is Isaiah 52:13-53:12. This one passage has by itself caused Orthodox Jews to expend many hours trying to define the servant as the nation of Israel rather than an individual. Yet the passage details the perfection of the servant, and perfection is not a quality of Israel.
Finally, the last ennead speaks to a time when the Holy Spirit will be active upon the earth, executing the will of God. The passage of Isaiah 61:1-3 is prominent, as Jesus quoted this passage in a small synagogue and attributed it to himself. In one part, Isaiah specifically uses the term Holy Spirit to speak concerning this time. This ennead finally speaks to the New Jerusalem and the New Earth, reflecting the very similar description found in the Revelation of John.
The point that this author seeks to establish is that Justin Martyr referred to Isaiah as the Fifth Gospel and rightly so. From the prophecies of the Messiah’s virgin birth, the Emmanuel God with us, and the suffering Servant, Isaiah has detailed God’s plan as found in the New Testament. New Testament Christians may enjoy focusing on those books written after the time of Christ, but they need to remember the Old Testament laid the groundwork for the first advent of the Messiah. Isaiah is not two documents but rather a unified plan of God to redeem all mankind.
 Gerhard von Rad, Old Testament Theology, vol. 2 (New York, NY: Harper and Row Publishers, 1965), p. 238.