According to the preface, how does a Sadguru receive service from devotees through the creation and study of books?

As explained in Chapter 0, a Sadguru can take two forms of service from devotees through books. The first way is by creating great works themselves, such as the Shri Dnyaneshwari. The second, more common method involves the Sadguru having their biographies written by their devotees. Through this act, the Sadguru accepts service from the writer who performs Guru service through their writing. Furthermore, service is accepted from the entire congregation of devotees who engage in the study and performance of Parayans of that biography, allowing them to achieve the purpose of their lives.