Thomas Stroik has published six articles on middles. I have found these articles useful in thinking about the middle, and I have also found them to resonate in certain ways with the analysis of the passive in Collins 2005 (in particular, the conclusion that the implicit argument in the middle is syntactically projected). So I have listed them all in one place. Stroik 2000 is particularly difficult to find. If you need it, please let me know.
Stroik, Thomas. 1992. Middles and Movement. Linguistic Inquiry 23, 127-137.
Stroik, Thomas. 1995. On Middle Formation: A Reply to Zribi-Hertz. Linguistic Inquiry 26, 165-171.
Stroik, Thomas. 1999. Middles and Reflexivity. Linguistic Inquiry 30, 119-131.
Stroik, Thomas. 2000. Syntactic Controversies. LINCOM. [Chapter 6: On Argument Demotion]
Stroik, Thomas. 2005. Book Review: Markus Steinbach. Middle Voice: A comparative study in the syntax-semantics interface of German. (Linguistik Aktuell; 50). Journal of Comparative Germanic Linguistics 8, 225-241.
Stroik, Thomas. 2006. Arguments in Middles. In Benjamin Lyngfelt and Torgrim Solstad (eds.), Demoting the Agent: Passive, Middle and other Voice Phenomena, 301-326. John Benjamins Publishing Company, Amsterdam.