According to the author of the Sai Satcharitra, were there other books or poems written about Sai Baba before this work?

Yes, Chapter 2 of the Sai Satcharitra explicitly mentions that other works about Sai Baba already existed. The author notes that Das Ganu described Sai's life in three chapters of his book, 'Bhakta-Lilamrut,' and a story told by Sai can be found in Chapter 57 of 'Santa-Kathamrut.' Additionally, the chapter references other sources containing Baba's leelas, such as Raghunath-Savitri’s Bhajanmala, the miscellaneous poems of Das Ganu, writings by Amidas and Bhavani Metha for Gujarati devotees, and a series of stories called 'Sai Prabha' being published in Pune.