According to the preface, what are the two distinct ways a Sadguru receives service from devotees through books?

The source material outlines two specific methods by which a Sadguru accepts service through literary works. As detailed in Chapter 0, the first way is when the Guru themselves creates profound spiritual texts, such as 'Shri Dnyaneshwari' or 'Shri Dasbodh'. The second method is when the Sadguru inspires devotees to write their biographies. The Guru then accepts service in the form of study and Parayans of these biographies, performed by both the author and the wider congregation of devotees. This highlights both direct and indirect forms of literary service.