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)
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)
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)