This is a reference to Kitāb ʿAlī, an early book that contained material transmitted by ʿAlī from the Prophet, which Jaʿfar al-Ṣādiq possessed and showed to his disciple Zurāra. Hossein Modarressi cites this reference to argue that ʿAlī was "the father" of the legal legacy of the Family of the Prophet and that the Prophet's teachings were passed down through the family from one generation to the next.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).