Reddit mentions of Ruling Passions: A Theory of Practical Reasoning

If you're interested in Harris's take on it in particular, I suggest looking at this blog post, and also follow the links to some philosophers' reviews of his book, The Moral Landscape. I'm glad Harris responded to his critics, though I don't think he rebutted the most important criticisms.

If you're interested in the underlying question about how ethics might be rationally derived, you could work your way through the SEP page on Kant's Moral Philosophy and investigate others from there. It's pretty dense though! Sidgwick's book that I mentioned above is good and very relevant if you want to trace through the history of these ideas.

If you want to skip to more recent discussion, Simon Blackburn has two books on the topic: Being Good is very accessible and meant to introduce the topics to non-philosophers; Ruling Passions is more technical but IIRC, Chapters 5 and 6 are very relevant to this exact debate and reasonably approachable.