I was interested in reading the following statement from H. G. M. Williamson’s statement in “Recent Issues in the Study of Isaiah,” in Interpreting Isaiah: Issues and Approaches, p. 21.
“The most noteworthy development in study of the book of Isaiah over the past two decades or so has been the rediscovery of the book’s unity. Prior to that, it was normal for commentaries to be written by different authors on different sections of the book, and for textbooks on prophecy to have separate (and separated) chapters on Isaiah of Jerusalem, “Deutero-Isaiah”, and so on. Nowadays, as we shall see, the picture looks very different.”
But before advocates of Isaianic unity get too excited, Williamson adds,
“This does not in the least mean, however, that scholars have reverted to a view that the book was all written by a single individual. While that position is still defended from time to time, it is more normal for a view of overall literary unity to be held in conjunction with a (sometimes quite radical) analysis of the history of the book's growth over two or more centuries with many hands contributing to it. Indeed, there are those who would now question whether, for instance, we should still think of a single author as being responsible for the bulk of chapters 40–55 (Deutero-Isaiah) or whether we should not rather envisage these chapters as the product of incremental growth by a larger number of writers.”
H. G. M. Williamson, “Recent Issues in the Study of Isaiah,” in Interpreting Isaiah: Issues and Approaches, ed. David G. Firth and H. G. M. Williamson (Downers Grove: InterVarsity, 2009).