The book can be devided into two sections. The first, longer, section is essentially the exploration of an empirical question, which is can we give an evolutionary account of the origins of the 'moral sense'? The second section is a look at what the consequences for moral theory might be of this empirical work, and is more conspicuously philosophical.
The evolutionary stuff is fascinating and well supported, but I want to actually read some of the empirical stuff he cites myself before I make a full assessment. In general his analysis of what a moral judgment can be taken for (and, hence, what sort of judgment we want to study the psychology of) was quite good and very instructive - although I think he might have made a mistake in focusing on the subject of the judgment (the judge) rather than the object of the judgment (that which is judged to be one way or the other). That might seem an odd criticism to make, given that this was a work on human biology, but I think that some of the conclusions he comes to are made inevitable by not focusing enough on the environment in which natural selection played out, but rather almost exclusively on the agents and their interpersonal reactions.
The philosophical section is of a very high quality. He divides it into two sections, based on two possible responses, called 'The Evolutionary Validation of Morality' and 'The Evolutionary Debunking of Morality'. I will take this opportunity to highlight the fact that he mentions G.E. Moore - as I have just listened to this charming lecture and so am quite enamoured with that great thinker right now. I don't think Joyce's discussion of G.E. Moore was satisfactory, although it was good to see him put paid to that bizarre attribution of the is-ought gap to Moore, which seems to be common purely because Moore made popular the term "naturalistic fallacy". However, his assaults on evolutionary ethics (Dennet is a big name who gets undermined here) in the rest of that first section, given his own empirical support, are very powerful indeed.
Then he goes and spoils it all in the last chapter. He thinks that what follows from his thesis is a sort of epistemological error theory, which he outlines in 'The Evolutionary Debunking of Morality'. Basically, we are never justified in believing any moral proposition is true - given that it is shown how we develop these beliefs with no reference to their truth - and we ought "cultivate an agnosticism" towards them if we are to be rational. However, like all moral skeptics he can't quite bring himself to really accept this and we have a postscript chapter called "Living with an Adapted mind", where he engages in a bit of philosophical doublethink; on the one hand saying we have to live with tough consequences while on the other hand assuring us that there's no practical outcome to "There is no reason to suppose any notion of morality accurate". Apparently having one's cake and eating it too is fitness enhancing. If the proposition 'There is no moral reason to believe you ought not, when the chips are down, murder your neighbour to your own advantage' has no practical importance then it is either because we are fortunate enough never to have our chips that far down, or because we have misunderstood its content.
All in all an excellent work, though, which I heartily recommend. Incidentally, the last chapter is headed with this excellent quote, which I think I will end on
If he does really think there is no distinction between virtue and vice, why, sir, when he leaves our houses let us count our spoons
- Boswell, Life Of Johnson