Hello Eoghan,
We do not know he was quoting the Book of Enoch. Calvin and Gill are of the opinion that Jude was quoting from a saying perserved in ancient tradition as an authentic saying of Enoch, held in high regard by the Jews of his day. Note Jude does not say, "Enoch wrote...", but "Enoch...prophesied..."
And from Thomas Manton’s commentary,
Jude, part of the BOT Geneva Series of Commentaries:
Ver. 14. And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of his saints.
The apostle urgeth another argument to imply the destruction of those seducers, and that is, the prophecy of Enoch. Whether this prophecy were written or not, the same Spirit that spake in Enoch inspired our apostle: if he received it by tradition, it is here made authentic and put into the canon*. The Jews have some relics of this prophecy in their writings, and some talk of a volume, extant in the primitive times, consisting of 4082 lines, called the Prophecy of Enoch; but that was condemned for spurious and apocryphal. Tertullian saith there was a prophecy of Enoch kept by Noah in the ark, which book is now lost. Be it so; many good books are lost, but no scripture. But most probably it was a prophecy that went from hand to hand, from father to son. Jude saith, ‘Enoch prophesied;’ he did not say it is written, as quoting a passage of scripture. But why should he rather produce Enoch’s prophecy, than a passage out of the authentic books of scripture, where there are many such to this purpose? I answer—....It was done by the providence of God, to preserve this memorial to the church....That the doctrine of the day of judgment is ancient, long since foretold.... (pages 289, 290, 291)
Notes
* Vid. Bez. et Estium in loc.
Hope this helps.
Steve