For those involved with the study of Kabbalah, the Sepher Yetzirah is, along with the Sepher Bahir, one of the key works of note within this genre of mysticism.
Further, to this is added the ten Sephiroth and their re.
Involved with both the Hebrew alphabet and numbers systems, it ascribes all major aspects of Creation (literally, formation by divine force) to three classes of Hebrew letters, with three numbered classes thereafter; the three mothers, the seven pairs, and the twelve elementals.
For those involved with the study of Kabbalah, the Sepher Yetzirah is, along with the Sepher Bahir, one of the key works of note within this genre of mysticism