I heard there's a story about a ritualistic man from Nashik in Chapter 12. Can you tell me who he was and why his story is important?

Yes, Chapter 12 introduces the story of an orthodox and ritualistic Agnihotri (a keeper of the sacred fire) from Nashik, whose surname was 'Mule.' His arrival in Shirdi is attributed to the strength of his past merits, as the text notes that one cannot stay there 'for even a moment' without such a foundation. The chapter presents his story as a 'very delightful story' that serves to illustrate the 'greatness of the Guru.' It is meant to be essential listening for devotees because it demonstrates how Sai works in mysterious ways, as Mule's original intention for his journey was different from the profound spiritual encounter he was destined to have.