The text mentions two types of service a Sadguru takes from devotees through books. Can you explain what those two types of service are?

The provided excerpt from Chapter 0 explicitly states, "The Sadguru takes two types of service from his devotees through books," but unfortunately, the text ends without elaborating on what these two types of service entail. The preceding sentences discuss the merits of writing and studying Guru biographies, but the specific two services mentioned are not defined or described. Therefore, based on the available information, it is impossible to identify or explain these two forms of service.