What are the two main types of service a Sadguru accepts from devotees through books, as explained in the preface?

The preface in Chapter 0 outlines two distinct ways a Sadguru takes service from devotees via books. The first method is when the Guru themselves authors a great spiritual work, with examples given like Shri Dnyaneshwari or Shri Dasbodh. The second method, which directly involves the devotee community, is when the Guru has their biographies written through their followers. The service is then accepted through the collective study and Parayans performed by both the writer and the congregation of devotees, creating a shared spiritual practice centered on the Guru's life.