Some references I’ve used:
The original book:
Fant, K. (2005). Logically Determined Design Clockless System Design with NULL Convention Logic. Hoboken: Wiley.
Fant, Karl M., and Scott A. Brandt. “NULL Convention Logic.” (n.d.): n. pag. Theseus Research Incorporated. 1997. Web. 27 July 2017. <http://www.theseusresearch.com/NCLPaper01.htm>.
Designing, optimizing, and background:
Smith, Scott C., and Jia Di. Designing Asynchronous Circuits Using NULL Convention Logic (NCL) Scott C. Smith and Jia Di. San Rafael, Calif.]: Morgan & Claypool, 2009. Print. Synthesis Lectures on Digital Circuits and Systems #23.
This one has some good sections on completeness: both input-completeness and completeness-completeness.
Please let me know if anything here is erroneous: