The first thing that has to be separated is Enochic tradition, the canonical Book of Enoch and the extant Book of Enoch. However, to do that, I am going to reference something totally unrelated as an illustration.
In Luke chapter 11 we see Jesus performing an exorcism and then making a strange reference. He says that "one greater than Solomon has come."
That's strange. Why would Jesus mention Solomon in connection to an exorcism? Well, because Solomon was an exorcist who enslaved demons. Wait, what? Where did that come from?
During the time of Jesus, the canon was not set. The Masoretic texts were not even fully wrought yet nor was the Babylonian tradition (out of which the Masoretc texts came) canon in Jerusalem or in Alexandria. In fact, the Septuagint was the most widespread form of the Jewish scriptures and there were at least three sets of canonical literature.
The great mistake people make is that they think the bible has come down in the exact same form that it was always in. Did you know that the Bibles commissioned by Emperor Constantine only had about 18-20 books in the whole thing considered canonical? The Epistle of Barnabas was one of them. Ever hear of that one? Probably not.
At the time of Jesus there were many texts that were considered authoritative. In fact, the Septuagint was the text of the core scriptures that Jesus references when he talks about scripture. Why? It was widely available and no one really spoke Hebrew back then; it was a ceremonial language like Latin to Catholics. They spoke Aramaic and Greek, and the Septuagint was the Greek text they could all read and understand.
Yet there were other texts that were not in the Septuagint that were also read and considered authoritative. Jude quotes from Enoch. Some say he is quoting Enoch, not the Book of Enoch. Okay. How did he know what Enoch said? It must have been in scripture somewhere. Anyone see a book quoting Enoch that Jude could have used in our canon today? No. It is not there. What does that mean? Well, it is like that mention of Solomon by Jesus that seems to come out of nowhere.
Jesus mentions the traditions outlined in a book called the Testament of Solomon. Flavius Josephus, the great Jewish historian, mentions a ring being used in a Jewish exorcism "such as was used by Solomon." This comes straight from the Testament of Solomon tradition.
Enoch, then is one of those referenced texts that Jesus and his apostles clearly knew about, read and regarded as scripture. There's just one problem...
The canonical Book of Enoch endorsed by the apostles and referenced by Peter and Jude (2 Peter 2 and Jude 1:6) appears to have been lost.
Remember Joshua references the "Book of the Upright." In fact this Sefer Ha Yashar is referenced in Joshua 10:13, 2 Samuel 1:18 and possibly in the 1 Kings 8, where it is found referenced in 1 Kings in the Septuagint version of the text, but not the Masoretic version of 1 Kings.
As an aside, the Masoretic texts are not older, but younger than the Septuagint. Interestingly, the oldest forms of the scriptures have Goliath at 4 cubits and a span. The Septuagint has him at the same height, but he has a growth spurt in the later Masoretic texts and is 6 cubits and a span. Other errors occur in the Masoretic texts, such as skipped lines that have to be filled in by using the Septuagint. So, yeah. The Masoretic texts have problems.
But back to the subject... the book that is often called Jasher (Sefer Ha Yashar) is obviously a canonical book that is lost. There are two extant Books of Jasher, but one is an attempt to recreate what the author thinks was in Jasher and the other is a Jewish Midrash. The point is that this was a canonical book that was lost and an attempt to recreate it, possibly from oral tradition.
The same is true for the Testament of Solomon. There is a canonical tradition of Solomon having power over demons through the use of a ring. Jesus references it and so does Josephus. The extant text is adulterated, though. It was widely read and is preserved thanks to early Christians. YES, this was a text used and considered canon by many early Christians. Unfortunately, they seem to have added to it, adding clearly Christian things to what was previously a Jewish text. Once a text becomes adulterated, one cannot say it is a divinely inspired text anymore. The rule of the church leaders was: if you can't accept part of it as original, you have to throw all of it out.
The same thing seems to have happened to Enoch. The overarching tradition of Enoch is canonical and clearly referenced by Moses, the prophets and the apostles. The original text is lost, or the original text is somehow partially lost with only fragments remaining, or the original text is completely lost with the oral tradition remaining. However, it is apparently not lost before the time of Jesus as it is referenced by the apostles.
The extant text, however, is adulterated. The extant text has sections that seem to be older (basis of verbiage and grammar) and other sections that seem to be younger. The problem is that it has been adulterated and you know the rule. If you cannot accept all of it as divinely inspired, you have to throw all of it out.
One problem there. The overarching tradition is still canonical and referenced by Moses, the prophets and the apostles. So you can't throw it out completely. If you do, you lack understanding of much of prophesy. What happened in Genesis 6 is the basis of God's later command to Joshua and later leaders of Israel to commit a genocide of the Canaanites. This was not explained in depth by Moses in Genesis because (according to tradition) Moses had the writings of the patriarchs before, including Enoch.
Now in the 5th century Julius Africanus responded to the criticisms of Celsus and Julian the Apostate by rejecting the Enochic tradition. Bene Ha Elohim (Sons of God) is a Hebrew idiom for angels. Benoth Adam (literally daughters of Adam) is an idiom for women born to human beings. Julius made up a new paradigm that really calls God a liar. He says that God didn't really mean God, but meant Seth and Adam really meant Cain. So the Nephilim were products of Sethites shacking up with Cainites. Problem is that Cain names his kids after God. He's a repentant sinner. Separation is not called for until later, by the way. Oh and... Seth's line all die in the flood, save Noah's clan -- so not so godly are they? (God says all flesh was corrupted)
Also, if you have two people from different families marry (Sethites and Cainites) the kids are not monstrous. Nephilim are monstrous according to both the canonical (remember Goliath, Og of Bashon, Anak and all that mess?) and non-canonical scriptures. Unfortunately, Augustine (in one of his many errors) accepts the Sethite contrivance and it becomes orthodox dogma for far too long. However the scriptures back up the Enoch tradition.
As far back as Eden. God tells the devil (as the dragon - serpent is likely mistranslated as the intent was probably dragon, as he is called in Revelation) that he will put enmity between the seed of the woman and thy seed. People often miss that there are two seeds there. The seed of the woman is the line of humanity leading ultimately to Jesus (and Jesus is called "the Seed of the Woman"). Who is the seed of the dragon? The Nephilim. This is why the Bible clearly says that there were Nephilim in the earth in those days (days of Noah) and after that - some survive the flood.
Yes. You read that right.
God is not complete in his wiping out of the Nephilim because he promised the devil that his seed would continue and there would be enmity between humanity and them. We see the Nephilim again in Numbers 13:33 and all through the Canaanite wars. The Nephilim are the reason for God's genocide against the Canaanites. And remember, Peter and Jude tell us the fathers of the Nephilim were bound in Tartarus. Some try to make a second incursion argument because they don't want Nephilim surviving the flood, but read what Genesis 6 says. Some do survive the flood. God leaves some alive to fulfill his prophesy to the devil.
Daniel prophesies their coming back in the prophesy of the statue with the feet of iron and miry clay. He tell us the group represented by the clay will try to mix with the seed of men but will not be able to. In order to attempt to mix with the seed of men, they must be other than the seed of men. Many scholars think this is talking about a resurgence of the Nephilim.
Understanding the Enochic tradition and the Angel View of Genesis 6 (which goes hand in hand with the Enochic tradition and is supported by Peter and Jude) is critical to understanding 80% of what happened in the founding of Israel and their wars and is also critical in understanding most of prophesy.
Now, the Book of Enoch did survive in Ethiopia. Unfortunately, it appears it was a second century, adulterated version that was canonized in the Ethiopian Church. The original without later ideas, grammar and other characteristics was not preserved in any tradition. Yet the overarching tradition of the Book of Enoch has remained, thanks to the hints here and there in the canonical text and the extrabiblical traditions until the discovery of the Enoch in Ethiopia and later fragments of Enoch in jars in the Holy Land.
So what does this mean? Simple. The Book of Enoch was a canonical work read, respected and considered authoritative by Moses, the prophets, Jesus and the apostles. It was not demonic fan fiction. It was not a false gospel (a gospel as defined in a textural sense is a story of the life of Christ, not an Old Testament book, so calling any Old Testament text a false gospel is an error by definition). It was a widely read, highly regarded, integral part of the religious canon at the time of the prophets and at the time of Jesus, just as the original work which was the basis of the extant Testament of Solomon was equally canonical in those times.
So, there were unequivocally other authoritative scriptures that we do not have in our canon today. Unfortunately, not all the scriptures that Jesus endorsed survived. Some were lost. Others were partially lost and adulterated forms survived, so they could not be canonized. That does not mean these biblically endorsed extra-biblical texts have no value. It means they must be read with discernment and should only be read by those with a strength of faith and the spiritual gift of discernment. Are they necessary for salvation? No. Can an understanding of the canonical part of the Enochic and Solomonic traditions enhance your understanding of scripture and your relationship with God? Yes. BUT, read without understanding and the spiritual gift of discernment they can create problems, just as reading the canonical texts in your off the shelf Bible without care and discernment can lead to error. But I would argue that these texts, because of their adulteration, pose a greater risk for the spiritually immature. A person of mature years and a mature spirit will do well with them, but others may find delving into these texts unwise.