Authorhip of the book is mysterious. Henry Spencer Ashbee, a book collector, writer & bibliographer notable for his particular focus on sexuality is suspected by some to be "Walter". He is very likely the compiler of the book's index, the provider of some editorial assistance & the person who saw the text into print.
Gershon Legman was 1st to link "Walter" & Ashbee in Legman's introduction to the 1962 reprints of Ashbee's bibliographies. The 1966 Grove Press edition of My Secret Life included an expanded version of that essay.
More recently, however, Ian Gibson's The Erotomaniac: The Secret Life of Henry Spencer Ashbee (2001, ISBN 0-571-19619-5) provides a detailed review of circumstantial evidence arguing that Ashbee wrote My Secret Life, presumably weaving fantasy & anecdotes from friends in with his own real-life experiences. Just prior to this, in May 2000, Channel 4 UK broadcast a documentary, Walter-The Secret Life of a Victorian Pornographer, which also claimed that Ashbee was Walter.