Theologian M. E. Baloyi reviews the arguments for and against single authorship of Isaiah.
M.E. Baloyi, "The Unity of the Book Isaiah: Neglected Evidence (Re-)Considered," Old Testament Essays 20, no. 1 (2007): 105-127
The evidence from Isaiah’s days, Isaiah’s activities, the book Isaiah, the other prophetic books, the New Testament, the nature of prophecy and tradition, as discussed in this article, together suggest that the book Isaiah is most probably a unity or that its final form suggests that the final editor intended it to be a unity.
The compilation hypothesis is found to be based on speculative foundations of unverifiable sources/fragments or ‘Isaiahs’, hence less probable. This criticism against the compilation hypothesis could also be raised against the redaction hypothesis as it is also speculative because it is based on speculative and unverifiable redaction texts at redaction stages and redaction processes.