In the Epistle of Jude, Jude makes a reference to Michael the Archangel disputing with the Devil over the body of Moses, and also to something that Enoch (Genesis 5:18-24) said:
Jude 1:9, 14-15
9 Yet Michael the archangel, when contending with the devil he disputed about the body of Moses, durst not bring against him a railing accusation, but said, The Lord rebuke thee.
14 And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of his saints,
15 To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him.
The first reference to the event involving Michael the Archangel, according to Origen (185-253 AD), Clement of Alexandria (150-215 AD), and others, comes from a text known as the Assumption of Moses. However, no extant copy of the Assumption of Moses contains a scene like the one described in verse 9, so it is assumed that such a scene, if it existed, is now lost.
The other reference, to something Enoch said, is a nearly direct quotation from the book of Enoch:
Enoch 1:9
9 And behold! He cometh with ten thousands of His holy ones to execute judgement upon all, and to destroy all the ungodly, and to convict all flesh of all the works of their ungodliness which they have ungodly committed, and of all the hard things which ungodly sinners have spoken against Him.
The book of Enoch is a forged, pseudepigraphal scripture, written many hundreds of years after Enoch died. It definitely uninspired, and should not be included in the Bible. Yet, Jude likely quotes from it. And, it was found amongst the Dead Sea Scrolls, and so it almost certainly predates Jude. What are we to conclude from this?
Firstly, Jude does not say that he is quoting Scripture. Rather, he seems to be citing well-known texts in the popular imagination of his audience, in order to make his points, using what is familiar to them. This would be as one today were to cite Shakespeare in order to make their point. Appealing to a text does not necessarily mean that it is real, or authoritative, or inspired by God, but rather, that it is useful for the point one is trying to make.
Secondly, there are other instances of extra-biblical sources being quoted in the New Testament, for the same purpose of making a point:
This, of course, does not mean that what the Greek poets wrote was inspired by God, and the Apostle Paul would be the first one to tell us that. Rather, their works were in the popular imagination, and therefore useful for making a point, because that is what the audience was familiar with.
In the New Testament, extracanonical books are cited, in order to make a point, by relating to what the audience would have been familiar with. This is a standard practice in writing and rhetoric, and does not mean that whatever source was quoted from was inspired or authoritative.