What does the author identify as the most supreme and certain form of written service for attaining the fruit of salvation?

According to the preface in Chapter 0, the best and most certain written service for achieving salvation is to write the biography of one's own Sadguru, undertaken either by the Guru's command or by inner inspiration. The author states that its spiritual efficacy is maximized when the devotee personally studies it regularly, performs hundreds of readings (Parayans) and weekly recitations (Saptahs) with a pure heart, excellent faith, and unwavering devotion. This service of body and speech is considered highly effective and does not interfere with one's worldly duties.