What are the two primary ways a Sadguru receives service from devotees through books, according to the preface?

The preface explains the two principal forms of service a Sadguru accepts from devotees via literary works. As detailed in Chapter 0, the first method is when the Sadguru creates profound spiritual texts themselves, citing examples like the Shri Dnyaneshwari or Shri Dasbodh. The second method involves the Sadguru inspiring devotees to write their biographies. Through this, the Sadguru accepts service from both the author who writes the biography and the congregation of devotees who study it and perform devotional readings, known as Parayans.