Om Sai Ram!
Dear vijjuhyd ji, may Baba help you in this work. Writing a book on Baba is possible only through His will and blessings. Pray to Him to inspire you and to make you an instrument of His will. That is what Hemadpant ji did before he wrote Shri Sai Satcharitra. In fact, he writes that Baba wrote the book through him. So, you must also think that way, and Baba is sure to help you.
I suggest that you go through the experiences of Sai devotees on this forum and other websites on Baba. The online forums are a rich resource where devotees share Baba's leelas in their lives. Hope this helps in some way.
Om Sai Ram!