Ihya Ulum al-Din · The Revival

Al-Ghazali's forty-book summa — law, worship, habit, and the diseases and cures of the heart — written to prove that the outer discipline and the inner path are one religion, not two. Its psychology of the heart-mirror (every act polishes or rusts) gave Islam its definitive manual of what the Safed kabbalists would call tikkun ha-middot, and Bahya ibn Paquda's Duties of the Heart — written in Arabic by a Jewish judge steeped in the same Sufi sources — carried the genre into Judaism a generation before him. The bookshelves were shared even when the creeds were not.