There are actually three books of Enoch. The second currently exists only in Old Church Slavonic, and theories about its origin place it anywhere from the 1st century BC to the 10th century AD (Nobody knows, in other words). The third is very late, and seems to have been written in Hebrew in Babylonia in the 6th or 7th century AD. I assume you mean so-called “1st Enoch.”
“1st Enoch” dates from the time between the Old and New Testaments (probably no earlier than the fourth century BC). Today the whole thing exists only in a Ge’ez (the language of Ethiopia) translation from Aramaic originals, but many fragments of it were found among the Dead Sea Scrolls.
1st Enoch is a pseudepigraphon–a book attributed to an author who couldn’t possibly have written it–and is said to be the work of Enoch, the son of Jared, who the Bible says “walked with God” (Genesis 5:21-24). It is a collection that includes many different kinds of material, but it is mostly known for its dreams and visions, in which Enoch is said to receive revelations about cosmology, wars between heavenly beings, and the last judgment.
The book seems to have been widely known in the last centuries BC and the first centuries AD. It was not, however, accepted by the Jews at the time of Jesus as part of the canon of Scripture.