Are there any other, lesser-known Guru biographies mentioned in the preface besides the 'Guru Charitra' and 'Sai Satcharita'?

Yes, the preface does acknowledge other Guru biographies, even if they are not as famous. It recalls a scholarly work by Shri Samarth Sadguru Tembhe Swami, also known as Vasudevanand Saraswati Maharaj. However, Chapter 0 notes this work did not achieve the same fame as the one by Saraswati Gangadhar. It also mentions hearing of a Guru biography written by Shri Samarth Sadguru Dattatreya Maharaj of Balekundri, but this work had not yet been printed or published. These examples illustrate that while other biographies exist, they were not as widespread or detailed as the primary two discussed.