It’s been probably 15 years since I read it, but I remember liking Relativism: Feet Planted Firmly in Mid-Air. The authors are Christian (actually, Beckwith has since become Catholic), but it’s really more of a philosophical book.
I also like Peter Kreeft’s A Refutation of Moral Relativism. Kreeft is a Catholic philosopher, but he writes the book as a Socratic dialogue between a relativist and a Muslim. I think he does this in order to show that it’s not just a Christian thing to have problems with relativism.
Sam Harris’s TED talk is a fairly potent dismantlement of moral relativism. Sam Harris, along with Dawkins, Hitchens, and Dennett, is considered to be one of the “Four Horsemen of New Atheism.”
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.