Philosophers debating free will have long understood that the term can be used in many ways, most of which are incoherent. Thus, advocates of "libertarian free will" (founded on the belief that free will requires indeterminism) have had to face the objection that indeterminate events in the brain would be expected to produce randomness, not freedom. And advocates of "compatibilist free will" (founded on the belief that some kinds of free will are compatible with determinism) have had to face other problems, including the one that many people find compatibilism intuitively implausible. Despite these difficulties, most leading philosophers (with a few important exceptions such as Galen Strawson, Derk Pereboom and Ted Honderich), have come to the conclusion that, if used cautiously, the term "free will" can be applied to human beings in a coherent, meaningful and true manner. One of the hard-won achievements of this 200 year old debate has been to separate out conceptions of free will that have a good chance of being coherent and even true, from those that are incoherent or probably untrue. It has been clear to all for many years that unsophisticated conceptions of free will are unlikely to stand up to philosophical analysis.
This 66 page text makes little attempt to contribute to the modern debate, but rather takes the easy option of attacking "the popular conception of free will" which, according to Harris "seems to rest on two assumptions: (1) that each of us could have behaved differently than we did in the past, and (2) that we are the conscious source of most of our thoughts and actions in the present". Of course, this popular conception gets a thrashing, because assumption (1) is ambiguous and assumption (2) is simplistic (interpreted to mean that we choose what to think before we think it).
Whether this conception is really popular is debatable. There has been research on what ordinary people believe about free will, and popular beliefs actually seem to be rather varied, but let us suppose that at least some people have a conception of free will resembling the one Harris attacks. For such people, the book may be useful. It is certainly much easier to read than the works of professional philosophers.
Harris has not refuted free will, but has mounted a ferocious attack on one rather naïve version of it. He doesn't seriously grapple with modern scholarship. Admittedly, he does briefly discuss two short texts from compatibilist philosophers Tom Clark and Eddy Nahmias. He merely dismisses libertarianism in a single sentence as not being "respectable" (page 16). He wins a cheap victory. Why should anybody be surprised if an unsophisticated "popular" view of free will can be knocked down?
If this easy-to-read 66 page tract stimulates people into reading more serious works on free will, this will be of value (they might start with Bob Doyle's comprehensive but readable book Free Will: The Scandal in Philosophy, 2011). If it lulls people into thinking that the problem is solved and free will does not exist, it may be a victory for obscurantism.