"Virtual History" has a very modern slant - its earliest subject is Charles I and the Civil War and 7 of the 9 essays relate to 20th-century events. As a reader with an interest in ancient and medieval history I was a little disappointed that there was not a little more variety. That does not undermine the quality of the writing at all, but some prior knowledge of the subjects covered is definitely helpful. To be fair, however, care has been taken to ensure that the subjects chosen are all fairly well-known events - the (English) Civil War, the American War of Independence, World War Two, the assassination of JFK for example.
Ferguson's introductory chapter is perhaps the best section of the book, being both excellent and informative. In it he takes a careful look at the development of counterfactual history over the ages, making reference to the literary world and to the development of science too. Some might find this section too dry, especially if their interest is more in the actual 'what if' scenarios presented in the following chapters. To me, however, this was one of the most engaging parts of the book, with plenty to consider.
One of the best essays is Andrew Roberts' "Hitler's England: What if Germany had invaded Britain in May 1940?" Here he explores plausible alternative courses based on contemporary attitudes, German strategies and British contingency plans. This is the kind of counterfactual history this book is really concerned with, rather than simply telling fictional stories. For this reason there seems to be little value in the last chapter, which attempts to tie together all the scenarios into a continuous narrative.
It is also worthwhile pointing out that "Virtual History" has an extensive list of endnotes, which means it is possible to follow up on many of the interesting points raised. At the same time, though, it would have been nice to have a bibliography or 'recommended reading' list. However, this is a minor flaw in what is generally an excellent read for anyone interested in 'what if?' history or even just looking for a new and imaginative take on what are well-covered subjects.