This is a reference to 400 books thought to be compiled by Jaʿfar al-Ṣādiq's disciples and including his legal opinions. Hossein Modarressi cites this reference in the course of his discussion on the extremely large number of reports and statements attributed to al-Ṣādiq, which, in the Shīʿī tradition, are held to have originated with the efforts of his many disciples.
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).