What does the preface say about the concept of Guru service and its different forms?

According to the preface, detailed in Chapter 0, Guru service is the primary path to attaining spiritual knowledge. It references Dnyaneshwar Maharaj, who taught that serving saints with one's entire being and without ego is how one dispels doubts. The text distinguishes between worldly service, like writing biographies of politicians, which is temporary and doesn't yield spiritual fruit, and spiritual service. The highest form of spiritual service is writing the biography of one's own Sadguru and performing regular recitations (Parayans) of it with selfless devotion, as this is said to grant the fruit of salvation.