What does Dnyaneshwar Maharaj teach about the nature of serving a Guru, as mentioned in the Satcharita's preface?

As recorded in the preface found in Chapter 0 of the Shri Sai Satcharita, Dnyaneshwar Maharaj provides profound guidance on serving a Guru. He emphasizes that service is the key to unlocking spiritual wisdom, calling it the "doorway" to the "treasure of knowledge." The service must be holistic, involving the surrender of one's body, mind, and soul at the Guru's feet. Crucially, as Chapter 0 explains, this service must be performed "without ego." Only then will the Guru impart the knowledge that enlightens the heart and dispels all doubts.