Jewish apocalyptic literature has long been recognized as an important feature of Jewish thought of the time, but it has only been recently that the literature is distinguished from the eschatology.
As one writer puts it, "apocalypse is a genre of revelatory literature with a narrative framework, in which a revelation is mediated by an otherworldly being to a human recipient, describing a transcendent reality which is both temporal, insofar as it envisages eschatological salvation, and spatial insofar as it involves another, supernatural world."
Apocalypses are new book pretending to be old ones, in that they are written as if they were by sages of the past: Enoch, Adam, Moses, Moses, Abraham, etc. And, these writings for the first time are considered to have two levels of meaning -- a surface meaning, and a deep, hidden meaning. Christian texts of about the second century C.E. call this second meaning "mystical."
The emphasis before this type of literature was incarnation. God came to meet humans. With apocalypse, it is the other way around. Humans are taken up to heaven to meet God. There are also dreams and visions reported, which had been part of Jewish writing for a long time. The ascent is the important part, for our purposes. It is the answer to the question "Where can God be found?" The traditional answer is "Among us", or more specifically, "in the Temple". The vision of God had always been important for the Jews -- think of Moses (burning bush) and Jacob (wrestling with God). But there was a shift in the intertestamental period, signalled by Ezekiel's vision of the chariot. It was a symbolic vision, and it was located outside the city of Jerusalem. Ezekiel's experience is one of the basic stories of Jewish rabbinic (Merkavah) mysticism; in fact, Merkavah means "celestial chariot".
But Ezekiel's vision lacked one thing -- the ascent into heaven. When ascent literature begins, you get something more than a discussion of the seeker's individual situation, or even the situation of Israel. There is the vision of the cosmos.
The other important contribution of the Jewish world to later mysticism is the establishment of a canon of texts, and the development of tools to make the texts continually alive to the community.
Note that there is another trend in ancient Jewish mysticism, called "Hekhalot", or throne mysticism. Both Merkevah mysticism and Hekhalot mysticism become important in Christian thought and in later Jewish thought, particularly the Kabbalah.