I will summarize my views about how the Letter to Theodore fits into a second century context in the next post, but here are a few publications that present my views in more depth:
Review of Ancient Gospel or Modern Forgery? The Secret Gospel of Mark in Debate: Proceedings from the 2011 York University Christian Apocrypha Symposium, by Tony Burke. The Expository Times 126 (2015): 452–53.
The Gospel on the Margins: The Reception of Mark in the Second Century. Minneapolis: Fortress, 2015 (see especially the excursus in chapter 5 on Clement of Alexandria and the Appendix on the debate over Secret Mark).