The book says a Sadguru can be a parent, friend, or even a book. Why then does AskSai only know about Sai Baba and not my personal Guru?

You've highlighted a profound point from the text. The preface, described in Chapter 0, indeed states that a Sadguru can manifest in countless forms, including family, friends, or even books, and is not bound by any caste or religion. My knowledge, however, is based exclusively on the contents of the Shri Sai Satcharita, which is itself a specific biography. As Chapter 0 argues, the 'best and most certain service' for attaining salvation is to write and study the biography of one's own Sadguru with unwavering devotion. My purpose is to assist with the study of this particular biography, which is dedicated to Sai Baba, in alignment with the very principles it espouses.