If Sai Baba is the Supreme Guru, why does the book talk so much about Rama? I'm confused about who I should be praying to.

Your question touches on a beautiful aspect of Sai's universal nature. Chapter 19 provides a wonderful personal account from the author to illustrate this very point. The author decided to continuously chant the name of Rama, and Sai Baba immediately confirmed this path by arranging for him to hear a song with the lyrics, "I have found the collyrium of Guru’s grace, my brother; I do not believe in anything without Rama." This story from Chapter 19 shows that Sai Baba encourages sincere devotion, regardless of the name used. The focus is on surrendering with an undivided heart, as Sai's grace supports any genuine spiritual effort.