A hypothetical book, which is asserted by a scholar of the Jesus Seminar
" version of the gospel of Mark. There is a rather poor case
for this book, and an even worse case for its authenticity.
All we have is one scholar who found a quotation (now lost) from Clement of
Alexandria which supposedly comes from this gospel. Further, we don't have any
reason to think the quote is valid or authentic. Worse, Clement apparently has
a history of accepting untrustworthy sources.
So, the Secret Gospel of Mark is something we don't have, quoted by a text we
don't have, by an author whose works are questionable.
Also see: Jesus Seminar, What's wrong with the Jesus Seminar?