A favorite of mine for a decade has been Randy Cornelius' book. I have used it as a part of teaching undergrad and grad courses in Canada, UK, Germany, and in Austria. If there is only one book to start what emotion research was and is all about, this is it - for me at least. As the book starts to be difficult to get, used might be the way to go. I just hope Randy can be convinced to do a second and updated edition!

Cornelius, R.R. (1996). The Science of Emotion: Research and Tradition in the Psychology of Emotion. Upper Saddle River: Prentice Hall.

More recent and certainly a good read is Psychology of Emotion: Interpersonal, Experiential, and Cognitive Approaches by Paula Niedenthal and her colleagues. This one has a less historical approach and might satisfy some readers more than the Cornelius book.

Niedenthal, P.M., Kruth-Gruber, S., & Ric, F. (2006). The Psychology of Emotion: Interpersonal Experiential, and Cognitive Approaches. Principles of Social Psychology series. New York: Psychology Press.

Psychology Press

Blackwell

Now in its second edition, Understanding Emotions, by Keith Oatly, Dacher Keltner, and Jennifer Jenkins, is a solid introduction to emotion theories and research with interesting sections on developmental and clinical aspects of emotions as well. They also talk about aspects of moral emotions, about compassion, or gratitude

Oatley, K., Keltner, K., & Jenkins, J.M. (2006). Understanding emotions (2nd ed.). Malden: MA: Blackwell.