A few key references

Maturana, Humberto R. (1970a) The neurophysiology of cognition. In Cognition: A Multiple View. Paul L. Garvin, ed. Spartan Books, New York, U.S.A. pp. 3-23.

Varela, Francisco J., Maturana, H. R. and Uribe, R. (1974) Autopoiesis: The organization of living systems, its characterization and a model. BioSystems 5:187-196.

Varela, Francisco J. (1975) A calculus for self-reference. Int. J. General Systems 2: 5-24.

Varela, Francisco J. (1979) Principles of Biology Autonomy. North Holland. New York, U.S.A. 303 pp.

Maturana, Humberto R. and Varela, Francisco J. (1980) eds. Autopoiesis and Cognition - The Realization of the Living. Boston Studies in the Philosophy of Science, Volume 42. D. Reidel Publishing Company, Dordrecht, Holland. 141 pp.

Maturana, Humberto R. and Varela, Francisco J. (1987) The Tree of Knowledge - The Biological Roots of Human Understanding. New Science Library, Shambala Publications, Boston, U.S.A. 263 pp.

Maturana, Humberto R. (1988) Reality: The search for objectivity or the quest for a compelling argument. Irish J. Psychol. 9: 25-82.

Mendez, Carmen Luz, Coddou, Fernando and Maturana, Humberto R. (1988) The bringing forth of pathology. Irish J. Psychol. 9: 144-172.

Varela, Francisco J., Coutinho, Antonio, Dupire, Bruno and Vaz, Nelson N. (1988) Cognitive networks: Immune, neural and otherwise. In Theoretical Immunology, Part Two. A.S. Perelson, ed. Addison-Wesley Publishing Co., New York, U.S.A. pp. 359-375.

Varela, Francisco J. and Coutinho, Antonio (1991) Second generation immune networks. Immunol. Today 12: 159-166.

Varela, Francisco J., Evan Thompson and Eleanor Rosch (1991) The Embodied Mind. MIT Press, Cambridge, U.S.A.

Ceruti, M. (1994) Constraint and Possibility. The evolution of knowledge and the knowledge of evolution. Gordon and Breach, Lausanne.

Mingers, John (1995) Self-Producing Systems. Implications and Applications of Autopoiesis. Plenum, New York, U.S.A.

Varela, Francisco J. (1995) Resonant cell assemblies. Biological research 28: 81-85.

Capra, Fritjof (1996) The Web of Life. A new synthesis of mind and matter. HarperCollins, London, U.K.

Ruiz, Alfredo B. (1996) The contributions of Humberto Maturana to the sciences of complexity and psychology. J. Constructivist Psychol. 9: 283-302.

Maturana, Humberto R. (1996) Ontology of Observing. The biological foundations of self consciousness and the physical domain of existence.
Maturana, Humberto R. and Verden-Zoller Gerda (1996) Biology of Love

Maturana, Humberto R. and Sima Nisis de Rezepka (1997) Human Awarness: Understanding the biological basis of knowledge and love in education.

von Glasersfeld, Ernst (1997) Distinguishing the Observer: An attempt at interprreting Maturana

Mariotti, Humberto (1999) Autopoiesis, Culture and Society

I think the most comprehensive list of references on cognition and enaction, and the 'official' Maturana bibliography can be found at Randall Whitaker's Observer Web site.