The Jewish tradition has always known that Moses received more on Sinai than what was written down. The concept of oral Torah — a living tradition of interpretation and supplementary revelation transmitted alongside the written text — is fundamental to rabbinic Judaism. But there's another tradition, older and stranger, that speaks of actual secret books.
The Book of Jubilees claims to be one of these secret books — the text of a second revelation given to Moses on Sinai, dictated by the Angel of the Presence. The Book of Enoch claims to be Enoch's own record of divine revelation, preserved and passed down through Noah and his descendants. The Dead Sea Scrolls community possessed texts they believed contained the true interpretation of scripture, hidden from the masses and revealed only to the righteous community.
Even within the rabbinic tradition, there are references to hidden texts. The mystical tradition of Jewish Kabbalah speaks of hidden wisdom accessible only to those who have prepared themselves through years of study and practice. The tradition of Merkavah mysticism — focused on visionary ascent to the divine throne — draws directly on Enoch's heavenly journeys.
None of this should be mistaken for conspiracy theory. Ancient religious traditions across the world maintained distinctions between exoteric (public, available to all) and esoteric (inner, available to those prepared for it) teachings. The Jewish tradition was no different. What texts like Enoch and Jubilees represent is the esoteric edge of that tradition — the deep theology that the public texts only hint at.
Whether or not you accept their claims to divine origin, reading them changes how you read the Bible.