According to the preface, what is the most effective form of Guru service through writing, and how should it be performed?

The most effective and certain form of written service that grants salvation is to write the biography of one's Sadguru. As explained in Chapter 0, this should be undertaken either by the Guru's direct command or through a powerful inner inspiration. To maximize its spiritual benefit, one must personally study the work regularly, perform hundreds of readings (Parayans) and weekly recitations (Saptahs) with a firm resolve, pure heart, and unwavering devotion. This devotional practice, which involves both body and speech, is considered the best service and does not conflict with maintaining a family or pursuing worldly prosperity.