According to the preface, what are the primary ways a Sadguru receives service from his followers through written works?

As detailed in Chapter 0, a Sadguru accepts two main types of service from his devotees through books. The first method is when the Guru creates great works himself, such as the Shri Dnyaneshwari. The second, and more relevant to community building, involves the Sadguru having his biographies written by his devotees. Through this second method, he accepts service from both the writer and the broader congregation of devotees who engage in the study and devotional reading, or Parayans, of these biographies. This practice allows many to participate in Guru service.