Are there any specific, comprehensive biographies of Sai Baba recommended for group study or Saptahs?

Yes, the text explicitly recommends a specific work for this purpose. Chapter 0 highlights the biography written by the late devotee Shri Govind Raghunath alias Annasaheb Dabholkar about Shri Sachidanand Samarth Sadguru Sai Baba Maharaj. This work, titled 'Shri Sai Satcharita', is described as a recent, sweet, inspirational, and grace-filled biography. It was published chapter by chapter in the Shri Sai Leela magazine and is presented as one of the few detailed, comprehensive works suitable for Parayans and Saptahs, similar in stature to the famous Guru Charitra for another saint.