According to the preface, what is considered the most effective and certain form of service through writing for achieving salvation?

The most effective and certain service in writing that can grant the fruit of salvation is to write the biography of one's own Sadguru. As detailed in Chapter 0, this should be done either by the Guru's command or through inner inspiration. The process involves not just writing but also personally studying the biography regularly, performing hundreds of readings (Parayans) and weekly recitations (Saptahs) with a firm resolve, pure heart, selfless love, and unwavering devotion. This type of service, which engages the body and speech, is considered the best path and does not interfere with one's family life or worldly responsibilities.