According to the provided text, what is considered the most effective and certain form of service through writing for attaining salvation?

The text suggests that while writing biographies of worldly figures or even multiple saints can be a form of service, the best and most certain path to spiritual fruit is to write the biography of one's own Sadguru. As explained in Chapter 0, this service, ideally undertaken by the Guru's command or through inner inspiration, involves not just writing but also personally studying the work regularly. This includes performing hundreds of readings (Parayans) and weekly recitations (Saptahs) with a pure heart, unwavering devotion, and selfless love. This form of service is considered superior because it is aimed at achieving the ultimate spiritual goal and can be done without interfering with one's family or worldly responsibilities.