The Koran is the holy book of Islam, and the most adored and beautiful book for all Muslims.
It was not actually WRITTEN by anyone. It is the pure word of God, passed down through the angel Gabriel, who visited the Prophet Muhammad (PBBUH) during the course of many years.
What is it that makes Koran so special and more holy than any other religious book (Muslims also believe the books of other religions such as the teachings of Moses and Jesus are holy) is the fact that the Arabic that is used in the Koran which dates bake centuries and centuries, has not changed one single word.
Where the other books such as the Bible have changed over time and been translated and reinterpreted, Islam's book: The Koran, is the purest form of Abraham's religion.
So if you are Christian or Jewish, you are following Islam but it has been changed in a way so that you have altered beliefs. But God will still love you, in Islam he will accept anyone so long as they believe in one true God.