You can sum up Harris' argument about free will quite succinctly: you are free to choose what you want, but you are not free to want what you want. In this essay (it's not really a book with just over 60 pages of actual text) Harris outlines the idea that free will is an illusion. Our actions have causes that are fixed by our past histories. In other words, if you went back in time and duplicated your life to the last second, you'd be reading this sentence thinking the exact same thing you are now thinking. You did not have any "choice" in deciding to read that sentence, your life history and the causal events behind that history lead up to you reading that sentence. This of course, has profound implications for many aspects of human behavior, particularly morality.
I am on the fence with Harris, as I sometimes think I'm a compatibilist (a term I didn't know until I read this book). Essentially, that means yes, our choices are caused by a particular past meeting a particular present, but if an individual makes that choice on their own, free from unavoidable external pressures, than that individual is "free" with regards to their larger environment. However, I could never avoid the nagging thought that this is just a shell game, and Harris calls it out as such (and calls out Daniel Dennett for his belief in it). I still think it's a valuable short-hand for psychologists and behaviorists to use, but ultimately, I am persuaded that Harris is right. Because the causal events around us lock us in to our behavior just as surely as gravity locks the Earth around the sun.
If one believes in the uncertainty of quantum mechanics, then free will is still an illusion as random chance replaces causality. If one believes in a soul, then one is still stuck with the fact that we act before we know what we are consciously doing, and that we are unaware and/or not in control of much of our behavior (e.g., cravings). With regards to morality, Harris focuses on risk and prevention, noting that retribution is rather pointless under a deterministic scheme, although behavioral modification is not (behavior can be caused, punishment can cause changes). He also briefly points out how conservative viewpoints seem to be more deeply founded on the idea of free will, ignoring the crucial role that luck often plays in determining one's life, state of mind, and actions.
Overall then, this is a very short, but powerful book. I would criticize it for perhaps not having enough examples, but then it wouldn't be the short punch Harris wanted it to be. It could also use more discussion of the neurological evidence regarding consciousness, as that is still a relatively new area of scientific inquiry and things could change. That said, with the current knowledge we possess, Harris makes a strong argument that the idea that we have free will is simply an illusion. Far from condemning us to a meaningless life, Harris points out that this could ultimately lead us to more fruitful, positive, and compassionate lives. If nothing else, that makes this a short argument worth reading.