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

According to the preface, the best and most certain form of written service for achieving salvation is to write the biography of one's own Sadguru. This act should be undertaken either by the Guru's command or through inner inspiration, coupled with a pure heart, strong faith, and unwavering devotion. As detailed in Chapter 0, this service also includes personally studying the biography, performing regular readings (Parayans), and encouraging other devotees to do the same, a practice that does not interfere with one's worldly responsibilities.