I must disagree here. There were no demons until after the flood. The demons are the souls of the deceased Nephilim. The "sons of God" are, in fact, fallen angels that left their first estate and had children with human women. The nephilim were human/angel hybrids and their souls are now the evil spirits and demons talked about in scripture and other literature. This is why demons show up after the flood and not before. Angels are angels, they don't die and therefore are always in that state, however they are either fallen or committed to God.
The "Book of Enoch" is not canonized scripture however, neither is any of the writings of Socrates, Leonardo De Vince, Plato, and they are taken at face value and are very informative and useful.
The "Book of Enoch" can be compared, or paralleled with "the Book of Jasher" and the "Book of Jubilee" These three compilations are used extensively when researching Biblical events. They parallel the Bible in astonishing ways. The book of Enoch is also quoted in the Biblical book of Jude:
Jude 1:14International Standard Version (ISV)
14 Enoch, in the seventh generation from Adam, prophesied about these people when he said,
Look! The Lord has come with countless thousands of his holy ones.
In my opinion, if the Bible quotes it, it must have some merit.
In this book, written by the only man that so touched God's heart that He took him away and he didn't die, is the account of the nephilim and the fact that these fallen angels set into motion a avalanche of corruption that affected "all flesh":
Genesis 6:12
And God looked upon the earth, and, behold, it was corrupt; for
all flesh had corrupted his way upon the earth.
and resulted in God wiping out all living things accept for Noah, his wife, his three sons and their wives. Noah's family was the only pure bloodline left. This is why God choose him and his family.
I wouldn't be so quick to throw out these books. They are a fantastic lens into the ancient times and, in the case of the Book of Enoch, some very interesting prophecy and windows into creation and the heavenly world. Enoch had a revelation not unlike that of John with his book which is the Revelation of Jesus Christ to John