Actually, remembering back, I think it was publisher demand. Dr. Hahn gave talks at parishes on his conversion; one was recorded and became a rather famous tape for St. Joseph Communications, "Protestant Minister Becomes Catholic." After it had sold some ridiculous number of copies, Fr. Fessio of Ignatius press approached him and asked him to do a book version.
campion,
We're actually saying the same thing from different ends so to speak. Fr. Fessio knew Hahn's book would sell well precisely because so many people were fascinated by his audio and VHS tapes.
By the way, Monsignor Ronald Knox published his autobiography, A Spiritual Aeneid, when he was only 26! I guess our Reformed friends would consider him an egotist for that?