Aside from the well-known works, what other, perhaps lesser-known or unpublished, Guru biographies are mentioned in the preface?

The preface, as detailed in Chapter 0, mentions a couple of lesser-known or unavailable Guru biographies. One is a detailed and scholarly work written by Shri Samarth Sadguru Tembhe Swami, also known as Vasudevanand Saraswati Maharaj. Despite its quality, it has not become as famous as other biographies. Additionally, Chapter 0 notes that there is hearsay about another Guru biography written by Shri Samarth Sadguru Dattatreya Maharaj of Balekundri. However, at the time the source was written, this particular biography did not seem to have been printed or published.