Why does Isaiah not mention himself after chapter 39? Why is it that after the 39th chapter of Isaiah that there is no more third person. In addition, if Isaiah composed this book why is it in third person to begin with? Are there three authors to Isaiah? Two of them coming in after the 39th chapter.
It is generally accepted that Isaiah chapters 1-39 were written by Isaiah, son of Amoz, who lived during the reigns of Kings Uzziah, Jotham, Ahaz and Hezekiah of Judah, although there were numerous later additions and changes to that portion of the book. Uta Ranke-Heinemann, in Putting Away Childish Things, page 230, dates chapters 24-27, the 'Little Apocalypse of Isaiah', to the late post-Exilic period. It is not unusual for an author to write about himself in the third person, and scholars accept the historicity of Isaiah and accept him as the author of the opening chapters of his book.

Chapters 40-55 are attributed to an anonymous author now known as Second Isaiah or Deutero-Isaiah, who lived during the Babylonian Exile. This author had a very different style to that of First Isaiah, who used brief, emphatic diction, compared to Second Isaiah's poetic style and more extensive vocabulary. 

Chapters 56-66 are attributed to another anonymous author now known as Third Isaiah or Trito-Isaiah. The style of Third Isaiah is similar to that of Second Isaiah, but differences of content and focus create a seam in the present book. 

Abraham Ibn Ezra, a Jewish scholar of Spain during the twelfth century was probably the first to raise questions about the authorship of the Book of Isaiah. It has also been proposed that the writings of II Isaiah and III Isaiah were originally separate books, but were subsequently appended to the original Book of Isaiah.