Based on the provided text, what constitutes a 'Guru's biography' and what are the two main ways it can be utilized by devotees?

As defined in Chapter 0, a Guru's biography is a reliable and true book that describes the Guru's incarnation, life stages from childhood to old age, their wonderful plays (Leelas), incomparable miracles, and the significant works of their incarnation. This text from Chapter 0 also outlines that these biographies, much like other spiritual knowledge texts, are used in two primary ways. The first is 'Kamya,' which is with desire for worldly gains like prosperity or relief from distress. The second is 'Nishkam,' which is with a selfless mind, seeking liberation from the cycle of birth and death, which is considered the superior approach.