Sefer Yetzirahספר יצירה

The Book of Formation — tiny (some versions ~1,300 words), ancient (dated anywhere 2nd–6th c., attributed to Abraham), and the most load-bearing text in the tradition: it invents the word "sephirot," declares the 32 paths, sorts the alphabet into 3 mothers / 7 doubles / 12 simples, and describes creation as letter-engineering on a spinning wheel of 231 gates. Equal parts cosmology, linguistics, and instruction manual — the golem recipes cite it as their source code.