Jude quotes a prophecy of Enoch in verse 14. That does not mean he is quoting from a book named "Enoch." It could have been common knowledge among the people of Jude's day that Enoch had prophesied that years ago.
An example of something like that would be
Acts 20:25. Here Paul quotes Jesus as having said, "It is more blessed to give than to receive." Where is that found in the Gospels? It's not. It was probably common knowledge among the people back then that Jesus had said that but it was not quoted by Matthew, Mark, Luke or John.