The Book of Enoch
I have seen a concerning trend among many Christians over the past couple of decades, and that is that some are deriving theology from extra-scriptural sources. I'm not talking about the apocrypha that is considered scripture by some. I'm talking about things that have never been considered canon. Not every ancient religious text is worthy of being taken seriously, even Hebrew or early "Christian" writings. There are any number of ancient books that I could list, but The Book of Enoch is the prime example since it appears to currently be the main "offender". As might be obvious, I am not a fan of this book. I will explain why in the following text.
First, about the book itself.
There are actually three books of Enoch, though 1 Enoch is typically what is considered as "The" Book of Enoch (and I will go with that herein). It is accepted that it was written between 300 and 200 BC, although parts were apparently written as late as the first century BC. It was written in Greek, with very early translations from Greek into other languages. It appears that the work is a collection of different authors and it is unclear if it originated with these authors or if they merely collected earlier writings and/or stories (or some of each). There are some clear contradictions with the canonical books of the Bible, seeming to indicate that the author(s) were somewhat ignorant of (some of) the scriptures.
Orthodox Jews have never considered the book to be canonical. The church, from the start, considered the book noncanonical as well. There were (and are) small splinter groups both among Jews and Christians that include the book as part of their "Bible". The general view has been that the book is folklore, myth, and/or fable. Orthodox Judaism actually considers it to be heretical.
As I noted, a few small fringe splinter groups among Jews and Christians have considered the Book of Enoch to be canonical. Also, there seems to be wide acceptance of the book among Mormons although it is not officially considered canon. Finally, ideas from it have been used in multiple modern Hollywood science fiction/horror/fantasy films and TV series. The very fact that these are the groups among which the Book of Enoch has gained attention supports the idea that it is worse than merely extra-Biblical - it is deceptive.
Could not there be some truth in the Book of Enoch despite its flaws? Almost certainly! Even the Master of Deception mixes truth with his lies. But that would require discernment to see the contradictions with the Bible. And those things that remain may still not be true. I really see no value in such an exercise. Considering how Biblically illiterate most people (even Christians) are, they would be much better served by becoming more familiar with the Bible than with external claims of truth from dubious sources.
How then should we approach this book?
Now, some would say that there is no problem with the use of extra-scriptural books. Aren't Bible commentaries an example of extra-scriptural books? I don't disagree. But I don't derive doctrine from extra-scriptural books. Perhaps non-canonical works can be helpful in understanding the meaning of original words better. For example, commentators (such as Adam Clarke) use extra-Biblical contemporaneous writings to determine the shades of meanings of words by seeing how those words were used elsewhere in the culture of the time. But that is different than allowing the content of these works to add to, or alter, our theology.
Paul instructs Timothy to command certain people to not devote themselves to myths. I don't know what myths he was specifically referencing, but they could very well have included the Book of Enoch. Paul mentions that such things do not edify the believers, and I've noticed that those who are open to the contents of the Book of Enoch tend to become devoted to it at the expense of simple faith in Jesus and walking in the Spirit. They are titillated by its contents and I've yet to see someone who holds it in high esteem that hasn't had their theology warped in some way. I'm not saying that the book is magically evil or any such thing. But I give it as much attention as I do to other sources of lies, such as the novels written by Dan Brown.2 I certainly wouldn't recommend that an unbeliever read the book since they lack the guidance of the Holy Spirit. Such people are more likely to be led into error than into truth.
Some would say that the book of Jude actually quotes the Book of Enoch. The reason for this thinking is Textual Criticism that assumes that a text that describes something is considered to be a derivation of older works that describe the same thing. While that may often be true, the assumption is flawed by denying the possibility of direct revelation from God, or ignoring that there may be an even older work from which both works derive. In this case, it could be that Jude and Enoch are both relating something from an even earlier work which has since been lost in the sands of history. These Textual Criticism assumptions are also why some people consider the account of the flood in Genesis to have come from the historically earlier Gilgamesh epic. Anyone who critically looks at it realizes that both Genesis and the Gilgamesh epic are nothing more than records of the same event and otherwise have nothing to do with each other. Another possibility is that Jude received direct revelation from God on the issues that appear to come from the Book of Enoch. In any event, all we know from Jude is that those parts of the Book of Enoch that agree with Jude are true. It says nothing about the truth of the rest of that book.
Summary.
I have focused on how Christians should approach the Book of Enoch (essentially it should be ignored), but this approach also applies to every person or writing that claims to be truth. We must be discerning and compare all statements against the Bible. But to do that we have to be familiar with the Bible in the first place. The Holy Spirit is the one who helps us to recognize spiritual falsehood, but He usually doesn't operate in a vacuum - we have to have a working knowledge of the Bible that He will use to inform us. In addition, if we recognize a falsehood but cannot state why it is false, how can we help anyone else who may be deceived? One need not be a Biblical scholar to do this - but being Biblically illiterate is of no help.
Sadly, those who are most unfamiliar with the Bible tend to also be those that are not walking in the Spirit. If you have neither the guidance of the Spirit nor an understanding of the contents of the Bible, paying attention to these extra-Biblical works is guaranteed to corrupt your understanding of spiritual matters. I've even met some people who are Godly believers whose theology has been, to some degree, warped by things such as the Book of Enoch. Unless you are led by the Spirit to read such things with discernment, avoid them and study the Bible instead.
For those who have already read extra-Biblical claims of spiritual truth (whether the Book of Enoch, the Book of Mormon, magazines, newspapers, movies, or anything else), it is important that you start studying the Bible so that your misunderstandings can be corrected.3 Anything in contradiction to the Bible (properly interpreted) is a lie, and anything that is simply not addressed in the Bible is either 1) a lie, or 2) an issue of such little importance that God didn't feel it necessary for us to know - and we dare not major in minor issues. Anyone who has not honed his discernment by regular study of the Bible and constant fellowship with the Spirit, is likely to eventually be led astray by listening to extra-Biblical claims of truth. It may not shipwreck your faith, but it could very well stunt your spiritual growth. You can eat the spiritually nutritious food of the Bible or you can eat the poisonous, empty calories of things like the Book of Enoch. Just remember that we will become what we regularly consume.
1 1 Timothy 1:3-4
2 Author of "The Da Vinci Code", "Angels & Demons", etc.
3 2 Timothy 3:16-17