The Book of Abramelin

The Book of the Sacred Magic of Abramelin the Mage: a German-Jewish frame-tale (Abraham of Worms) teaching an 18-month (in Mathers' shorter French copy, 6-month) retreat of prayer and purification to attain "Knowledge and Conversation of the Holy Guardian Angel" — after which the famous letter-square talismans become usable. The retreat-structure — devotion first, powers after — is its genuinely kabbalistic heart.