Can writing biographies be considered a form of Guru service, and what are its limitations according to the text?

Yes, the preface in Chapter 0 explicitly states that writing biographies is a form of Guru service. However, it draws a critical distinction. Writing about worldly figures like politicians or historians is a service that is limited by space and time and does not produce spiritual fruits because it is often tied to self-interest. Writing the biographies of saints, on the other hand, is a form of Guru service that does yield spiritual fruits. Even so, the text notes that this type of service is considered "one-sided or partial" and may be insufficient on its own to attain the ultimate spiritual goal.