I don't know too much about Derek Parfit yet but he seems to want to combine Kantianism and consequentialism, at least according to the introduction. He also seems to think that the "Formula of Universal Law" can be saved in a form like "Everyone ought to follow the principles whose universal acceptance everyone could rationally will."(pg. 22)
Without having read the guts of Parfit's arguments I can say at the outset that I'm going to have difficulty accepting this kind of moral foundation. I'm not convinced that I have to behave in a way that any "rational being" would will. I don't think that there is a rational being, or anything such as rationally willing something. Parfit has chapters on Nietzsche, and perhaps he deals with this issue, but I don't see how Parfit is going to be successful. If I don't believe in rationality, and I don't believe in rationally willing anything, I don't see how I'm going to agree with Parfit.
In the introduction we find: "As Parfit acknowledges, his reliance on a primitive and "indefinable" notion of "reasons", and his concomitant commitment to the existence of irreducibly normative truths, both about reasons and about morality, makes his view a version of what Korsgaard has called "dogmatic rationalism". As such, it would be resisted not only by Kantian constructivists like Korsgaard but also by proponents of some very different meta-ethical outlooks, such as various forms of naturalism and non-cognitivism"(pg. 25)
The introduction explains: "Naturalists hold that normative facts can be reduced to natrual facts. Non-cognitivists hold that normative claims, despite their importance in human life, do not function as statements of fact at all."(pg. 26)
I suppose if I had to classify myself at the moment I'm some sort of naturalist. It will take a lot for me to agree that there are irreducibly normative truths. I'm too much of a nihilist to think there's truth at all half the time -- not all the time, just half the time. But, I think the book holds a lot of interest.