Prinz's book is another, though particularly sophisticated, attempt to rehabilitate William James' theory of emotion in philosophy. It draws upon Prinz's wide-ranging and thorough knowledge of recent work done in cognitive neuroscience and marries this with his avowed commitment to empiricist principles.
Basically, the trick Prinz plays is as follows:
James claimed emotions were the perception of certain feelings or sensations (not perceptions or judgments of states of affairs in the world that bring about the feelings, as the cognitivists claim). For James, we have these sensations in response to states of affairs in the world, and then we perceive those sensations (we perceive our cold sweat and raised heartbeat as fear). It is our perception of this sensation that IS the emotion.
Now, the upshot of James's theory is that our emotions are arational or irrational irruptions in an otherwise rational life.
Cognitivists have contested this. Authors such as Solomon in The Passions, Kenny in his now classic Action, Emotion and Will, Goldie in The Emotions, the cognitivist psychologist Richard Lazarus in his Emotion and Adaptation, Nussbaum in her (much maligned) Upheavals of Thought, Gabriele Taylor in her Pride, Shame, and Guilt and so on have all argued, contra James, that emotions are constituted by judgements or evaluative beliefs (or intentional feeling in Goldie's version).
Prinz seeks to rehabilitate James but in a manner that doesn't imply that emotions are arational or irrational. He wants to revive James's theory though made more robust through the adoption of some of the insights of cognitivism--chiefly Lazarus's version. Prinz does this by imbuing those gut reactions with psychosemantic content by rendering Lazarus's notion of core relational themes sub-personal. You might find this move of Prinz's to be either an ingenious piece of philosophical theory or a peice of hopelessly flawed metaphysics. Phil Hutchinson has argued forcefully in his Shame and Philosophythat the latter depiction is most accurate (and he then offers an alternative framework for understanding emotion: "world-taking cognitivism"). Wherever one stands, Prinz's book should not be ignored. Just, don't believe the hype!