Biblical Archaeology Review has an analysis by several scholars that conclude the text was not forged by Smith. See here.
Here’s a bibliography of earlier studies on Secret Mark.
Morton Smith’s Original Studies
Smith, Morton. Clement of Alexandria and a Secret Gospel of Mark (Harvard UP, 1973)
Smith, Morton. The Secret Gospel: The Discovery and Interpretation of the Secret Gospel According to Mark, 3rd ed. (Dawn Horse, 2005)
Pro-Authenticity
Brown, Scott G. Mark’s Other Gospel, (Wilfrid Laurier University Press, 2005)
Meyer, Marvin W. Secret Gospels: Essays on Thomas and the Secret Gospel of Mark, (Trinity Press, 2003)
Anti-Authenticity
Jeffery, Peter. The Secret Gospel of Mark Unveiled: Imagined Rituals of Sex, Death, and Madness in a Biblical Forgery, (Yale UP, 2006)
Carlson, Stephen C. The Gospel Hoax: Morton Smith’s Invention of Secret Mark, (Baylor UP, 2005)