What is considered the most effective form of written Guru service for attaining salvation, according to the Shri Sai Satcharita's preface?

According to the preface, or Prastavana, the best and most certain written service for achieving salvation is to write the biography of one's own Sadguru. This service should be undertaken either by the Guru's direct command or through inner inspiration. As detailed in Chapter 0, this act should be performed with a firm resolve, pure heart, excellent faith, selfless love, and unwavering devotion. Furthermore, the service extends to personally studying the work, performing regular readings like Parayans and Saptahs, and encouraging other devotees to do the same.