Sri Guru Charitra

Many are the blessed souls who, by daily devout study of this work, have earned the grace of Lord Dattatreya and his unerring guidance respecting to their Perfect Masters."....

...."It suddenly struck Panduranga that it was the sacred book, which Datta directed him to read and he gladly received it. He was pleasantly surprised to know that the book was personally given to his uncle by Sree Sai Baba of Shirdi, an avatar of Lord Datta. By deligent and devoted study of it, he blossomed into Sree Sadguru Rangavadhutha Maharaj of Nareswar
http://www.saibharadwaja.org/books/srigurucharitra/readbook.aspx?book=43&chapter=0