Book of Enoch?
The book of Enoch is interesting, to be certain, and we can assume that both Jude and his readers were familiar with it. But while it's interesting and sheds light on the thoughts of first-century Jews and Jewish Christians, it would be unwise to take it as authoritative in the same way that we regard Scripture. Here's a partial list of reasons:
- It was not recognized as inspired/canonical by any early Christian author. It doesn't appear in any of the lists of texts that became known as the New Testament.
- It has not been transmitted as reliably as the New Testament texts (this dovetails with #1; few Christians made copies of it because they didn't consider it Scripture).
- It's probably not written by Enoch (it's never referenced in the Old Testament, only the New, but if it was actually written by Enoch it would be the oldest book in the Old Testament).
I think it's always safe to ask questions and to get a better picture of the beliefs of the early church. But in this case, I wouldn't want to draw serious doctrine from the Book of Enoch. Even Jude references it only in passing, and even then only as a story that he assumes his audience is familiar with. We only know this story from the Book of Enoch, but we don't even know for certain if that was Jude's only way of knowing that story. Many historical documents have been lost to us over the years, but we believe that we have the Bible that God wants us to have.