I follow Richard Beck's blog, Experimental Theology, and I've read three of his other books. I think Unclean: Meditations on Purity, Hospitality, and Mortality is his best so far: it is thoughtful, insightful, practical and has a more accessible style than his first two. I'm guessing it was written for a wider audience; his early works were the fruit of Beck's own academic psychological research.
In Unclean, Beck expounds on the emotion of disgust—how it is a basic part of human development and how it functions as a protective measure both biologically and culturally. He explores where the disgust reflex comes from: it develops early, but it's based on culture and learned. Babies will put anything in their mouths; it's by watching adults and by trial and error that they train their disgust response. It's protective—a boundary psychology—because reflexively avoiding body fluids, dead animals or rotting food keeps us healthy and safe. Even the social extension of disgust is meant to protect the integrity of the group. This is why disgust doesn't just extend to bodies and food; its link to culture means it's broader than that, and tends to override rational thought. (If you saw Inside Out, and remember the character of Disgust, you've got this.)
Beck is truly interested in how disgust affects our ability to love each other--how dealing with the world around us through the lens of clean/unclean affects our ability to behave like Christ. He starts with Jesus' discussion with the Pharisees in Matthew 9:13, when they challenge him about eating with sinners (thus becoming unclean). “Go and learn what this means,” Jesus tells them, “I desire mercy, not sacrifice.” Beck interprets this as Jesus challenging to the purity codes of the Jewish law, and by extension, all of our purity codes.
“Purity,” as a metaphor, can easily become twisted. Disgust is about preventing contamination, and that goal has several components. First is contact: you're contaminated by touching something dirty. The second is dose insensitivity: even the tiniest bit of contaminant ruins the whole thing, like a few drops of urine in a glass of wine. The third is permanence: drop a bug in a pitcher of juice. Gross, right? Now boil the liquid. It's clean, but people in experiments still wouldn't drink it. Once ruined, it couldn't be purified. The fourth, and most powerful to me, is negativity dominance: the pollutant has all the power. Those drops of urine contaminate the wine; if you put a few drops of wine in a cup of pee, it wouldn't have the same effect.
If you can't make the connection from that paragraph to experiences in your own life (especially if you're a churchgoing Christian or work in social activism), I suggest you read the whole book and reconsider. Frankly, I recommend Unclean to anyone who finds this topic at all interesting, or who has struggled with the divide between body and spirit/mind, or with the concept of the Incarnation, or with being told they were “dirty.” Disgust is a broad concept, and it influences us in ways we rarely consider consciously.