It looks like you’re asking for a (academic or analytical) on the phrase "index of rahasya."
However, "Index of Rahasya" is not a standard published work or widely known text in mainstream religious or academic databases. The word rahasya (रहस्य) is Sanskrit/Hindi for — often used in titles of Tantric, Puranic, or Upanishadic texts (e.g., Devi Rahasya , Rahasya Traya Sara , Rahasya Upanishad ).
To help you properly: