If you rate a work of history like this strictly in terms of how informative is it, then this is more like a 3.5 or 4-star book, as it is informative, though in a repetitive and general kind of way, with a careful omission here and there.
I'm a little more critical than that, though. Truly I have never read a historical analysis where the evidence and details were so remote from, and in some cases contradictory to, the thesis provided. Honestly, there's far too much detail in the book that bears no relevance to the book's purpose or argument. (There's a crucial difference between "evidence" and "details.") And yet, Horn barely mentions important details like Bacon's rebellion in 1676 or the chartering of the Royal African Company in 1662. I suppose, being the president of the Jamestown Whatever Society, it was incumbent on Horn to strike while the 400-year-anniversary iron was hot. But the other problem is that most of the evidence provided stands in stark contrast to his silly "democracy started in 1619" argument.
Like many historians, Horn asks too much of the past. To look back at this moment of 17th century English people looking to make enormous profits and transplant English society abroad, and then to expect them to attempt any sort of democratic government, is pretty absurd. Again, Horn joins many historians, especially public/pop historians, in this teleological tradition; the assumption is that, since we today are Americans, and since everything about America is "representative" and "democratic" (hogwash, of course), and since Virginia was the first colony with the first governing body in what would randomly become the USA more than 150 years later, then we today must be able to connect those dots to say that that governing body was "democratic" or "representative" or whatever. It's a type of fiction based on a prostitution of facts and causality.
Here I'll try to address the two key elements of Horn's intention: explore the ironies and paradoxes of a single year wherein the first "representative" governing body was established and the first Africans arrived in Virginia.
The first argument is fairly easy to dispel. As suggested above, I outright reject the teleological tradition on which many national histories (not just American) are based. Jamestown was, plain and simple, a for-profit venture with no intention of establishing a democratic or representative government. In fact, every shred of evidence that Horn provides clearly shows this; he specifically states that, for the next 150 years, economic and political power continued to be consolidated in the hands of the few wealthy planters and in plain disregard for common laborers, servants and slaves. Horn's entire argument is based on the notion that Sandys, whose political origins were rooted in the House of Commons, had some gripes with monarchism and believed he could found a parliamentarian government in Virginia. To whatever extent Sandys's political differences with King James even mattered here, the point is moot considering two basic points: (1) nobody else involved in the founding and operation of the House of Burgesses shared Sandys's so-called "democratic" premonitions, and (2) Horn uses the term "representative" extremely loosely and selectively. Whenever he discusses the Burgesses' representative capacity, he uses terms like "settlers" or "people." The fact is, however, that the Burgesses were specifically hand-picked by the wealthy plantation owners and no one else, and the Burgesses were there to represent the planters' and Company's interests exclusively. The terms "colony," "plantation" and "company" were all interchangeable, and the Burgesses were little more than a public front to the private interests of Company shareholders and wealthy planters. Sandys's "headright" system, wittingly or not, only exacerbated both the exorbitant death rate among servants and the consolidation of economic and political power among planters. The Burgesses, planters and Company men were all influential in London and had a direct role in getting laws passed in that parliament that permitted them to force orphans, debtors, the unemployed, and other unfortunate English people onto ships headed for Virginia, none of whom would ever be represented in the House of Burgesses. Not to mention the exploitation of the Irish. The Virginia Assembly was merely a transplanted English institution, giving private planters and shareholders a concerted and legitimized public voice.
Nor does Horn properly distinguish between colonial PR and actual daily life in Jamestown. He mentions some of the PR pieces, like Thomas Harriot's Briefe and True Report, but in other moments he treats what were merely advertisements for Virginia as accurate description of Virginia. Yes, many English intellectuals tried to spread word of the many subsistence and cash crops that could be grown in Virginia. But in actual life, particularly around 1619, it was almost entirely tobacco because that's what was making the land-owners, the company, and the crown all of the money. The laboring class hardly had a thing to eat that wasn't stolen from the natives or from each other, and Sandys had to try to compel by law the planters to grow edible foods, with mixed results at best. (The price of tobacco was on the verge of decline due to an oversaturated market, which eventually became more of an incentive to grow more diverse crops than "feeding the workers" seemed to be.)
Now for slavery. Here Horn actually impressed me insofar as he was willing to draw our attention to an existing historiographical debate: can we really call it "slavery" already in 1619 or shortly thereafter? I tend to think that there are different reasons to say "yes" or "no" to that question, and I think Horn says "yes" for the wrong reasons. Again in rather teleological fashion, Horn's assumption, like many historians, is that African = slave. In that vein, Horn does a decent job explaining why we should accept that Africans in Virginia were always slaves, because they were targeted increasingly by VA Assembly laws and because they were always essentially bought and sold. Left out of that argument, however, is the fact that many, many English and Irish men, women and children were also bought and sold in Virginia during this time. In fact, there's a decent book, titled White Cargo, that details much of this, plus primary source records. VA Assembly laws targeted servants and tenants, whose contracts would restart if they were caught breaking a law (such laws included swearing, drinking, and even stealing boats and oars). Between the criminal code and the life expectancy (for some periods an 80% death rate), extremely rare was the servant who lived to see the end of his contract and reap the rewards in the form of a plot of his own land. It wasn't until the Company's and then planters' efforts to compel would-be servants to go to Virginia met resistance in England that we start to see a concerted effort to bring Africans into Virginia between 1640-1660 (plus a civil war in England). And for much of this time, the differences between a "servant" and a "slave" in Virginia were nominal at best, and only later were increasingly racialized through deliberate economic and political efforts. It wasn't until around 1670 that we see a law concerning "intestate" goods specifically used the term "negro" and "slave" interchangeably. In short, if we want to assume that, in order to be slaves, they must also be Africans, then there's something to Horn's argument. But if we're willing to strip the term "slave" of it's uniquely American historical baggage and not use later concepts of race as a teleological prism, then I think Horn's argument is a waste of time.
Some historians, in line with Ira Berlin, like to argue that 17th century Virginia was a "society with slaves" rather than a "slave society." This, too, strikes me as a hair-splitting argument based on the assumption that they must be Africans to be considered slaves. And even then, it's an odd argument, one not worth making by my standards.
Going back to my point that this colony was always, from it's initial charter onward, a for-profit venture, I think it is worthwhile to strip this context down to it's most motivated players who had the most agency and purpose. As historian Andrew Fitzmaurice details nicely in Humanism in America, these early modern "explorers" drew from a tension between "public good" and "private gain," and concepts like "commonwealth" were instrumental in licensing these pirates and privateers to pursue their own wealth at will, as long as they had "the crown" in tow. We literally have state-sponsored piracy in this case, with knighted figures like Sir Walter Raleigh and Sir Francis Drake doing the crown's bidding. So rather than wondering at what point Virginia shifted from being a "society with slaves" to a "slave society," or if it's fair to consider the first handful of Africans in Virginia to be slaves, it's more worthwhile to think about slavery as an economic issue and to think about what problem it was meant to solve. And I don't see any way around the fact that this is a matter of supply and demand. Black or white, African or Irish or English, the Company and wealthy planters had a labor demand, and they found ways to meet it through their representatives in both London and in Virginia's House of Burgesses. Perpetual servitude and slavery were the results. To call all of that the result of a "democratic" or "representative" government is silly. Up until around 1670-1680, my take is that we have both white slaves and black servants, white servants and black slaves, and all were made so in an English class system that was uniquely revised in a new, polyglot North American environment through an undemocratic parliamentarian body -- the House of Burgesses -- that was essentially a continuation of its class-based progenitor in London.
Finally, though, I'd like to take up Horn's thesis and presuppose that his argument about "representative government" in Jamestown is in fact true and accurate, that we today are indebted to the likes of Sandys and the original House of Burgesses for planting the seed of "representation" in America. I don't think you'd have to twist many arms these days to get people to admit that "representative" government in America today is, like in 1619, controlled by powerful private interests who have steadily consolidated economic and political power at the expense of common, individual voters. Just look at a group like ALEC, for example. You could point to many instances wherein, much like colonial Virginia and like the early modern era of Humanists in general, we continue to operate under an arbitrary and arguably feckless or non-existent distinction between public good and private gain, and yet we continue to heed the words of those who say "representative" without being clear as to who, in fact, is being represented.
But Horn is right about one thing for sure. New England gets too much attention, and Virginia not enough, in the way US History is taught.