The followers of Kabbalah, a branch of Jewish mysticism, explore hidden meanings e.g. in the Torah in order to achieve a more spiritual relationship with God. Kabbalah was originally an oral tradition but from around the 13th century onward it was stored in literature. (Signs & Symbols, 2008, p.174)
Read or download: Zohar Or: Book of Splendor
Read online: Sepher Yezirah (also Sefer Yetzirah.Hebrew, Sēpher Yəṣîrâh = "Book of Formation," or "Book of Creation," ספר יצירה) (translated by Isidor Kalisch, 1877), or Sepher Yetzirah (translated by Wescott, W.W. (1887).