There are two major portions of the book. Some people have labeled them Isaiah I and Isaiah II. The first part contains a lot of history of Israel and the confrontation they had with other countries. They were threatened and beseiged, and Isaiah predicted that they would eventually fall to the Babylonians. Chapters 1-35 give a review of the kings of Israel and of the threats.
Chapters 36-39 provide a description of an interlude between the worsening apostasy and the exile. Hezekiah was a good king and his reign was renouned. This passage includes Jerusalem's miraculous deliverance from Sennacherib.
Chapters 40-66 describe the call to repentance to the people in exile. God through Isaiah promises relief, restoration, and redemption to a broken and distressed nation.
Some scholars believe the two portions are written by different people. It is amazing that the predictions and prophesies are very accurate. On the other hand, that is the definition of prophesy.
Isaiah could be called the Gospel according to Isaiah. It has several passages that describe the birth of Jesus, his ministry on earth, and his suffering and death. These passages occur in both sections of the book, so if, indeed, there were two authors, they were surely of one mind.
Isaiah is the most quoted prophet in the New Testament. Jesus quotes him many times and Paul, Peter and the Gospel writers all quote him, too. Isaiah is the most prolific of the prophets. The importance of this book to Jews and Christians cannot be overestimated. Read it; it is worth your time.