The preface says a Sadguru can be 'in the form of a book' or can be one's 'mother, father'. Which of these does the author say provides more spiritual fruit, and how does he compare the two paths?

In the preface, presented in Chapter 0, the author lists several forms a Sadguru can take, including 'in the form of a book' and one's 'mother, father, siblings, friend, husband or wife.' However, after listing these possibilities, the text does not proceed to compare them against each other or rank them in terms of the spiritual fruit they might yield. The primary focus of the preface shifts to the act of serving a Guru through writing a biography, particularly that of one's own Sadguru, without offering a direct comparison between the different forms a Guru might take.