Could you elaborate on the two distinct ways a Sadguru accepts service through books, as mentioned in the preface?

Certainly. The preface in Chapter 0 explains that a Sadguru accepts service via books in two ways. The first way is by creating profound spiritual texts themselves, such as the Shri Dnyaneshwari by Shri Dnyaneshwar or the Shri Dasbodh. The second way is by inspiring their devotees to write their biographies. In this case, the Sadguru accepts service not only from the writer but also from the entire congregation of devotees who engage in the study and devotional readings (Parayans) of that biography. This is exemplified by the Shri Sai Satcharita, written by Annasaheb Dabholkar.