From the Treasury and our Synod's website:
Isaiah son of Amoz is considered to be the greatest of the writing prophets and is quoted in the New Testament more than any other Old Testament prophet. His name means “Yahweh [the Lord] saves.” Isaiah prophesied to the people of Jerusalem and Judah from about 740 B.C. to 700 B.C. and was a contemporary of the prophets Amos, Hosea, and Micah. Isaiah was a fierce preacher of God's Law, condemning the sin of idolatry. He was also a comforting proclaimer of the Gospel, repeatedly emphasizing God's grace and forgiveness. For this he is sometimes called the “Evangelist of the Old Testament.” No prophet more clearly prophesied about the coming Messiah and his saving kingdom. He foretold the Messiah's miraculous birth (Is 7:14; 9:6), his endless reign (Is 2:1–5; 11:1–16), and his public ministry (Is 61:1–3), but most notably his “Suffering Servant” role and atoning death (52:13—53:12). The apostle John's description of Isaiah, that Isaiah saw Jesus' glory and spoke of him (John 12:41), is an apt summary of Isaiah's prophetic ministry.
[It was in honor of his commemoration that we sang about his augural vision in "Isaiah, Mighty Seer" during today's liturgy]