In the vast ocean of Tantric literature, few texts are as precise, influential, or geometrically minded as the Sat Chakra Nirupana (षट्चक्रनिरूपण). For serious practitioners of Kundalini Yoga, scholars of comparative religion, or anyone seeking the original source behind the modern "seven chakra system," this 16th-century Sanskrit treatise is the ultimate reference.
Download the PDF as a reference, but keep a print copy of The Serpent Power nearby. The diagrams of the chakras with their petal-counts and mantras are far more useful on paper than on a screen.