This one is not open source - you'll need to find it in your nearest library or purchase it online. If you are interested in learning more about TL Theory, this is where it all began.

Citation (APA): Mezirow, J. (1991). Transformative dimensions of adult learning. San Francisco, CA: JosseyBass

Open source article published in the Sage journal, Adult Education Quarterly.

Citation (APA): Hoggan, C. D. (2016). Transformative learning as a metatheory: Definition, criteria, and typology. Adult Education Quarterly, 66(1), 57-75.