Describe the two main ways a Sadguru accepts service from devotees through books, according to the preface.

The preface explains that a Sadguru takes two types of service from devotees through literary works. The first way is when the Sadguru creates great spiritual works themselves, with examples given like the Shri Dnyaneshwari or Shri Dasbodh. The second method, as detailed in Chapter 0, is when the Sadguru inspires devotees to write their biographies. These biographies are then used for devotional study and readings, known as Parayans, by both the writer and the wider community of devotees. Through these two avenues, the Sadguru accepts service in the form of study and devotion from the entire congregation.