Why is the Book of Enoch not in the Bible?
The Christian can trust in the historical authenticity and inspiration of the Bible. Its integrity as the Word of God is above question. Those who wish to add or take away from the revealed Word of God should be rejected.
There are many excellent books that deal with the scripture and how we got the Bible in its present form. They can be obtained through your local Christian bookstore.
The main issue with the Book of Enoch is that most scholars believe it was written around 300 BC. The Biblical Enoch was the great-grandfather of Noah, who built the ark around 2300 BC, so more than 2,000 years separated the translation of Enoch and the writing of the book that bears his name.