It has been proposed that there was an Isaianic school or guild responsible for the additions to the original work. This attractive proposition presents difficulties, for it suggests a line of disciples extending over two centuries, not mentioned in biblical sources, and coming into prominence only at the close of the Babylonian captivity.
On the other hand, there can be little doubt that Isaiah's words and those of other eighth century prophets were studied and restudied, particularly during the Exile. In the Exilic period, when words of doom must have assumed new and deeper significance because they were fulfilled, those students or disciples who pored over these oracles perhaps were led to new insight, understanding and hope.
Out of their insight could have come Deutero-Isaiah with its message of redemption, utilizing some Isaianic terms ("Holy one of Israel") and developing ideas drawn from Isaiah of Jerusalem. The message, appended to the great prophet's work, completed the pattern of doom and hope of a remnant with the joyous announcement that what was promised was about to be fulfilled. Beyond such speculation we cannot go at present.
http://www.infidels.org/library/modern/gerald_larue/otll/chap18.html
The fact that the opposite view is nowadays almost universally accepted requires us to enlarge upon what is to be found in this passage. The idea of an oath by God as the most solemn form of assurance is found in prophecies of judgment by Amos and Isaiah, and is taken up by the Deuteronomic redactor of the book of Jeremiah, who draws elsewhere on the conception which his school held of the promise of the land of Israel as an oath to the fathers.
A similar idea is also found in the book of Ezekiel. On the other hand it is possible to trace how from the time of Deutero- Isaiah this usage.......
