Paramahamsa Yogananda in his "Autobiography of A Yogi" revealed some things about this immortal yogi but not much, only to tell the wider audience about the founding father of the tradition that was named "Kriya Yoga". A little later Babaji dictated three books to two of His devotees, Yogi Ramaiah and Neelakantan. These three books have been translated into English by Babaji's Kriya Yoga Order of Acharyas in "The Voice of Babaji: A Trilogy on Kriya Yoga".
Several Kriya Yogis have testified about Babaji's darshans. Among them are: Yogiraj Lahiri Mahasaya, Swami Sri Yukteswar Giri, Paramahamsa Hariharananda Giri of Karar Ashram in Puri, Swami Satyeswarananda Giri (Sanskrit Classics) to name a few. Their stories have been publicly available already.
However there is also a whole group of other devotees who have received Babaji's direct blessings and have been in actual and direct communication with Him. I've come to know through my own experience since I was child that Babaji many times guides spiritually orientated and mature persons without making Himself known. So there probably are a lot of people who receive His guidance but are not aware of it.
For example Prema Baba Swamiji - Dr. Donald Schnell - and Sivakami Om Anandi (both Americans) have publicly told of their experiences with Babaji and other Mahasiddhas - the Great Perfected Ones.
In his book "The Initiation" Prema Swami narrates his spiritual journey and his encounter with Kriya Babaji Mahavatar in 1996 in India.
These are just a few examples of Babaji's direct guidance and blessings. It is not the purpose of this site to try to convince anyone of Babaji's existence or His work, I am convinced that He makes it Himself to those who are ready.
Since 2007 the author of this site has been communicating directly with Kriya Babaji Mahavatar and the purpose to tell it openly is that Babaji's personal actual guidance is available to all who turn to Him.