Nesse RM: Evolutionary explanations of emotions. Human Nature, 1: 261-289, 1990.
    This classic article has been widely cited.  Its main contribution is recognition that describing functions for emotions does not offer a full evolutionary explanation because one emotion has many functions and different emotions have many shared functions.  Instead, an evolutionary explanation for an emotion needs to describe the situation that shaped the emotion, the adaptive challenges posed by that situation, how selection shaped systems to detect the presence of that situation and express the emotion in those situations, and how the characteristics of the emotion increase fitness when expressed in those situations.

Nesse RM, Ellsworth PC. Evolution, emotions, and emotional disorders. American Psychologist, 64(2):129-39, 2009.  
This article offers an updated approach to the evolution of emotions, with the emphasis on the importance of appraisal theory, and the sobering conclusion that natural selection is most unlikely to have shaped well-defined emotions or dimensions of the sorts that  researchers have so long searched for.

Nesse RM. Evolutionary origins and functions of emotions. In: Scherer K, Sander D, editors. The Oxford Companion to Emotion and the Affective Sciences. Oxford: Oxford University Press. p. 159-64, 2009.
A clear simple overview

Nesse RM. An evolutionary perspective on panic disorder and agoraphobia. Ethology and Sociobiology;8:73S-83S, 1987.

Nesse RM: An evolutionary perspective on psychiatry. Comparative Psychiatry 25:575-580, 1984.

Nesse RM: What is mood for? Psycholoquy 2: Issue 9.2, November 24, 1991.       Replies to commentaries:
My earliest clear statement about the evolutionary origins of mood

Nesse RM: What good is feeling bad? The evolutionary utility of psychic pain. The Sciences, 30-37, Nov./Dec. 1991.

Marks IM, Nesse RM: Fear and fitness: An evolutionary analysis of anxiety disorders. Ethology and Sociobiology 15:247-261, 1994.

Nesse RM: Computer emotions and mental software. Social Neuroscience Bulletin 7(2): 36-37, 1994.

Nesse RM, Berridge K Psychoactive drug use in evolutionary perspective. Science, 277: 63-65, 1997.

Nesse RM: What Darwinian medicine offers psychiatry. Evolutionary Medicine, W. R. Trevathan, J. J. McKenna and E. O. Smith. New York, Oxford University Press, 1999.

Nesse RM Emotional Disorders in Evolutionary Perspective. British Journal of Medical Psychology 71:397-415, 1998.

Nesse RM Proximate and evolutionary studies of stress and depression: Synergy at the Interface Neuroscience and Biobehavioral Reviews,23 (7): 895-903, 1999.

Nesse RM Is depression an adaptation? Archives of General Psychiatry, 57: 14-20, 2000.       Letters in response and reply

Nesse, RM Is grief really maladaptive? (Review of The Nature of Grief, by John Archer) Evolution and Human Behavior 29: 59-61, 2000.

Nesse, RM: Natural selection and the regulation of defensive responses. Evolution and Human Behavior, 26(1) 88-105, 2005.
The smoke detector principle, with the math.  Best source for this work.

Nesse RM, Bhatnagar S, Young, E: The evolutionary origins and functions of the stress response, The Encyclopedia of Stress, Second Edition, Academic Press, NY, 2007.