According to the preface, what are the two ways a Sadguru accepts service from devotees through literary works?

The text outlines two distinct methods through which a Sadguru receives service from his followers via books. As explained in Chapter 0, the first method is when the Sadguru creates great literary works themselves, with examples like Shri Dnyaneshwari or Shri Dasbodh being cited. The second method involves the Sadguru having their biographies written by their devotees. In this scenario, the Sadguru accepts service through the study and devotional readings, or Parayans, of these biographies, performed by both the author and the broader community of devotees. This allows the writer to perform Guru service and gain merit by serving God in humanity.