What are the teachings of Guru Nanak on the interplay between personal effort and divine grace, according to this book?

The texts you've provided from the Shri Sai Satcharita do not mention Guru Nanak. However, Sai Baba offers a powerful metaphor in Chapter 19 that beautifully illustrates the relationship between human effort and divine grace. He advises a devotee, "You start doing the exercises (Zor); leave the worry about milk entirely to me; I am standing behind you with a bowl." This illustrates that a devotee must sincerely put in the necessary effort, but the ultimate nourishment and results are provided by the Guru's grace. One must be alert and perform their duty, trusting that the Guru will provide for their well-being.