Based on the preface, what are the different types of biographical writing and what are their respective spiritual benefits?

According to the preface in Chapter 0, there are several types of biographical writing, each with different outcomes. Writing about worldly Gurus like leaders and writers is considered a worldly service with limited, time-bound importance and no spiritual fruits. Writing the biographies of many saints is a step up, considered a form of Guru service that does yield spiritual fruits, but is still seen as partial or one-sided. The most beneficial, according to the author, is writing the biography of one's own Sadguru, which is described as the best and most certain service for attaining the ultimate spiritual goal of salvation.