Isaiah was a prophet (holy man). God appointed Isaiah to warn the people in Judah. The people were evil. So, they should ask God to forgive them. Isaiah lived before Ezekiel and Jeremiah.
Isaiah described the future of many countries (Isaiah chapters 13-24). But especially, he warned his own people in Judah and Israel (Isaiah chapters 28-31).
But this book also contains many happy words. Isaiah wrote about a child who would have no human father (Isaiah 7:14). This child would become king (Isaiah 9:6). He would rule fairly (Isaiah 32). And the people would be glad (Isaiah 35). So, God will comfort his people (Isaiah 40). This child would be God's servant (Isaiah 42). But people would not accept him. God's servant would die so that God will forgive us. Then, God's servant would live again (Isaiah 53). Everyone must trust him (Isaiah 55). God will forgive the people who confess their evil deeds to God (Isaiah 59). So, God himself will rescue his people (Isaiah 63).
We now know that Isaiah wrote these words about Jesus.
The Book of Isaiah is in the Old Testament section of the Bible.