Based on the preface, what are the different forms of 'Guru service' and how do they rank in terms of spiritual efficacy?

The preface, as detailed in Chapter 0, outlines a hierarchy of 'Guru service.' The lowest form is worldly service, such as writing biographies of great but secular individuals, which yields no spiritual fruit. A higher form is writing the biographies of many saints, which is a form of spiritual service but is described as one-sided or partial. The highest and most certain service that leads to salvation is writing the biography of one's own Sadguru. This act, especially when commanded by the Guru or inspired from within and followed by personal study and recitation, is considered the most spiritually potent form of service.