What are the two types of service a Sadguru takes from devotees through books, as described in the source material?

As explained in Chapter 0, a Sadguru takes two primary types of service from his devotees through books. The first is when the Guru creates great works himself, with examples given like Shri Dnyaneshwari or Shri Dasbodh. The second method is when the Guru has his biographies written through his devotees. This allows the Sadguru to accept service in the form of study and Parayans from both the writer and the broader congregation of devotees. This second form is exemplified by the 'Shri Sai Satcharita,' which was written about Sai Baba for his followers to study and perform devotional readings.