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

The preface, in Chapter 0, outlines two distinct ways a Sadguru takes service from devotees in the form of books. The first method is when the Sadguru creates profound spiritual works themselves, with the text citing examples like 'Shri Dnyaneshwari,' 'Shri Nath Bhagavata,' or 'Shri Dasbodh.' The second method involves the Sadguru having their biographies written by their devotees. In this case, the Sadguru accepts service not only from the writer but also from the wider congregation of devotees who engage with the work through study and devotional reading, known as Parayans. This establishes a tradition where many devotees have written prose and poetic biographies of their Gurus.