The book called 1 Enoch is not one book. It is five works of different dates gathered under one name: the Book of the Watchers, the Book of Parables, the Astronomical Book, the Book of Dreams and the Epistle of Enoch.
The Book of the Watchers expands the two obscure verses of Genesis 6 about the sons of God and the daughters of men into a full account. Two hundred angels bind themselves by oath on Mount Hermon, take human wives, and beget giants who consume everything and turn on humanity. Worse than the marriages is the teaching: the Watchers hand over metalwork and weapons, cosmetics and dyes, root cutting, astrology and the reading of signs. Corruption enters the world as illicitly disclosed technique. Enoch is asked by the fallen angels to carry a petition on their behalf, writes it, and brings back the refusal.
The Astronomical Book sets out a solar calendar of 364 days in twelve months, with the sun passing through gates in the east and west. That scheme divides evenly into 52 weeks, so every festival falls on the same weekday every year, and it is irreconcilable with the lunar reckoning that governed the Jerusalem temple. Communities that held it were therefore keeping different holy days from the priesthood.
Aramaic fragments of every part except the Parables were found among the Dead Sea Scrolls, which established that the material is genuinely ancient and pre-Christian, and made the absence of the Parables, the section that speaks most of a heavenly Son of Man, one of the standing questions of the field.