Enoch (1 Enoch), the book of Jasher (book of the Righteous) and the book of Jubilees are all supported by scripture in the Bible and also agree with one another.
As it was in the days of Noah, so it will be at the coming of the Son of Man, Enoch was written for our generation.
The Essenes thought it was important, Jude quoted it directly, Genesis chapter 6 talks about the flood, 2 Samuel 1:18, Joshua 10:13 call the book of Jasher by name, I don't know enough about Jubilees to say anything on it beyond that it tells a similar story.
The tl;dr of all of them is as such:
200 angels(called watchers because their job was to watch and subtly guide humanity) rebelled against God and made a pact with one another to come down to earth and have children with human women. (their reason for doing this was to corrupt the seed of man to prevent the redeemer from being able to be born, it obviously didn't work)
Their offspring were the giants, incredibly unbelievably large in the first generation, getting smaller after that.
The watchers taught various technologies to humanity, among them being how to make implements of war, how to make cosmetics, how to perform abortions, astrology, etc.
The offspring of the fallen angels, called giants, nephilim and mighty men trampled the whole earth, killed, ate and raped humans and committed every kind of degeneracy.
From the actions of the watchers, their offspring and the sins of the sons of Cain all flesh on the earth was corrupted, man, beast, fowl and every creeping thing. There are other texts found among the dead sea scrolls that indicate that the fallen angels were mixing different animals dna and human dna and that the offspring of those experiments were literal monsters.
As judgment for all this evil God passed the judgement of the flood, made the watchers witness their offspring kill each other, he also had the watchers bound for 70 generations of mankind at which point some of them would be let out to lead mankind astray again, the souls of all their offspring are bound to the earth becoming disembodied spirits when they die, what we know of as demons.
In the last of days when the watchers are let loose and knowledge is increased in the earth the rebellion will be brought to its climax and the world will be cleansed by fire the second time, 2 Peter agrees with this.
I think it is inspired, when reading them there is a strong spirit of truth to them, I think they were removed because being forearmed with knowledge against these things would have prepared a great many more people for the schemes of the sons of darkness, but that the elect in these days will recognize the truth anyway.
Jesus is Lord