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

The preface in Chapter 0 outlines two distinct methods by which a Sadguru receives service through literary works. The first way is when the Sadguru creates great spiritual works themselves, with the text providing Shri Dnyaneshwari and Shri Dasbodh as examples. The second method involves the Sadguru having their biographies written by their devotees. In this case, the Sadguru accepts service in the form of study and Parayans (devotional readings) performed by the writer and the congregation of devotees. The Shri Sai Satcharita by Annasaheb Dabholkar and the Brihat Guru Charitra by Shri Saraswati Gangadhar are cited as prime examples of this second category.