Can you explain the difference between worldly and spiritual Guru service as discussed in the text?

The preface in Chapter 0 distinguishes between two types of Guru service. Worldly service, such as writing biographies of great politicians or writers, is considered limited by space and time. Its fruits are not eternal because it is often rooted in self-interest, and therefore it does not yield spiritual benefits. In contrast, spiritual Guru service, like writing the biographies of saints, does yield spiritual fruits. The text from Chapter 0 states that writing the biography of one's own Sadguru is the best and most certain path to the ultimate spiritual goal, especially when done with selfless love and unwavering devotion.