Isaiah 1:1 (NKJV)
1 The vision of Isaiah...which he saw...
The book of Isaiah is the recording of a single vision, according to the words of 1:1. While brimming with all kinds of judgments and promises upon and to multiple nations and peoples, and while collected and then recorded over the reigns of multiple kings, Isaiah is a single message of the glory of God in His salvation of the world.
In the vision, Isaiah sees what God was doing, why he was doing it, and how it was all playing out under His perfect sovereign control to reveal the glory of His name.