Enoch was the great-grandfather of Noah. He is referenced in the New Testament of the bible by Peter and Jude.
Though The Book of Enoch had been in wide circulation in the period leading up to the first century A.D., and despite its evident popularity, having been viewed as scripture and quoted in many other books of the day, in the late first century, the spiritual leader of the Jews at that time, decided upon a strict canon of scripture, one which eliminated Enoch and other books.
Perhaps the writings of Enoch were rejected from the canon because church leaders found the story of angels having sex with humans too fantastic. The book of Enoch gives much detail to the unholy alliance mentioned in Genesis Chapter 6.