According to the preface, what are the different types of biographical writing, and which is considered the most spiritually beneficial?

The preface outlines a hierarchy of biographical writing as a form of service. Writing about worldly Gurus is a form of public service, but its fruits are not eternal and are rooted in self-interest. A higher form is writing the biographies of many saints, which yields spiritual fruits but is still considered partial. As explained in Chapter 0, the most certain and beneficial service that grants the fruit of salvation is to write the biography of one's own Sadguru. This should be done by the Guru's command or through inner inspiration, and be accompanied by regular study, readings (Parayans), and weekly recitations (Saptahs) performed with pure faith and selfless love.