Can you explain the different types of Guru service through writing and their spiritual effectiveness?

According to the preface in Chapter 0, there are different levels of Guru service through writing. Writing biographies of worldly Gurus is considered a worldly service with limited, non-eternal fruit, often rooted in self-interest. A higher form is writing the biographies of many saints, which does yield spiritual fruits but is still considered partial or one-sided. The highest and most certain service is described as writing the biography of one's own Sadguru and diligently studying it through Parayans and Saptahs. This form of service, when done with a selfless (Nishkam) mind, is considered superior and capable of granting the ultimate spiritual goal of liberation.