Al-Buni (d. 1225) is perhaps the most famous—and controversial—figure in this field. His works, especially Shams al-Ma'arif al-Kubra (The Sun of Great Knowledge), are almost entirely built on Ilm al-Huruf and talismanic squares. While many mainstream scholars warn against his practical applications, understanding his theoretical framework is unavoidable. A word of caution: Many PDFs claiming to be "Ilm al-Huruf complete PDF" are actually excerpts from al-Buni’s work. Approach these with ritual purity and proper guidance.
Before you download that "ilm al-huruf pdf," a word of serious caution. In mainstream Islamic orthodoxy (Ahl al-Sunnah), practical application of Ilm al-Huruf—particularly using the Shams al-Ma'arif—is considered Haram (forbidden).
This secretive 10th-century brotherhood produced the Rasa'il Ikhwan al-Safa (Epistles of the Brethren of Purity). Epistle 3 and 4 are dedicated entirely to arithmetic, geometry, and the science of letters. They viewed letters as the spiritual "quantities" that define the material world. A scanned PDF of the Rasa'il is a treasure trove for any serious student.