According to the preface, what different forms can a Sadguru take? Is it always a person?

The Preface in Chapter 0 offers a very broad perspective on the nature of a Sadguru, stating that one is essential for salvation and can manifest in numerous forms beyond just a person. The text explicitly mentions that a Sadguru can belong to any caste or religion and can be "in person or in the form of a book, whether they are saints, deities, mother, father, siblings, friend, husband or wife, known or unknown." This highlights the core principle that the Guru's grace is what matters, irrespective of the form through which it is delivered.