What are the different forms of Guru service mentioned in the preface, and which one is considered the most effective?

The preface outlines several forms of Guru service. As noted in Chapter 0, one form is worldly service, such as writing biographies of great politicians or writers, but this is considered limited and does not yield spiritual fruits because it is often rooted in self-interest. A higher form is writing the biographies of many saints, which does yield spiritual fruit but is still considered partial. The text suggests that the best and most certain service for attaining salvation is to write the biography of one's own Sadguru, study it with faith and devotion through recitations, and encourage others to do the same. This form of service is considered supreme because it is a selfless act aimed at spiritual liberation.