Can you explain the two forms of service a Sadguru accepts from devotees through the medium of books?

The preface describes two ways a Sadguru takes service from devotees through written works. The first method is when the Sadguru creates great spiritual works themselves, with Shri Dnyaneshwari and Shri Dasbodh cited as examples. The second method involves the Sadguru having their biographies written by their devotees. In this case, as Chapter 0 explains, the Guru accepts service from both the writer and the congregation of devotees who engage in the study and Parayans of these biographies.