The preface mentions several Guru biographies. Can you identify which ones were considered less famous or were unpublished at the time of writing?

Certainly. Chapter 0 discusses several biographies with varying levels of prominence. A Guru biography written by Shri Samarth Sadguru Tembhe Swami, also known as Vasudevanand Saraswati Maharaj, is mentioned as being scholarly but not having achieved the same level of fame as the one by Shri Saraswati Gangadhar. Additionally, the text notes that while Shri Samarth Sadguru Dattatreya Maharaj of Balekundri was said to have written a Guru biography, it did not seem to have been printed or published at that time, making it inaccessible to the public.