In this book, a descendant of George Washington does his best to construct an underwater tunnel to link Britain and America by train. It is the 1970s, and the British Empire is still thriving, thanks to 1) the Spanish never having reconquered Iberia from the Muslims (and thus the English discovered and took over all of the Americas) and 2) the British having won the (attempted) American Revolution, during which George Washington was executed for treason. Now his descendent, Augustine "Gus" Washington, is an engineer working on the trans-Atlantic tunnel, hoping to restore his family's honor.
Harrison's alt-history is clever, but vastly underused. The few peeks we get at what the world would be like if the British Empire had been even larger and lasted even longer are intriguing, but Harrison gives us little more than some vague sketches. For example, more Native Americans would have survived, which makes sense given the fact that the British government was generally more concerned with their welfare than was the United States. In addition, the World Wars apparently never happened, and it seemed implied that this was due to the sheer hegemony of the British Empire. These sorts of things were intriguing and at least somewhat plausible to me, since I did a lot of graduate work in history on the British Empire. However, like I said, Harrison gives us only the merest hints.
The steampunk-esque (though this book predates the steampunk movement, I think) technologies in the book are often intriguing as well - in this alternate world, we have airships, steam power, and analog computers coexisting side-by-side with electronic and nuclear power, so it's simultaneously more and less advanced than the world we live in. However, Harrison has a tendency to get bogged down in wearisome and minute descriptions of these technologies, at the expense of other elements of his story, which correspondingly suffer.
And that's what my main complaints are - though some of Harrison's descriptive passages were very good, overall characterization, plot, conflict, and dialogue were fairly weak. (In particular, as at least one prior reviewer pointed out, Washington's character is very flat.)
The basic premise of this book could have been made into a really great, dramatic story, but Harrison's usage of it falls short. So in sum: Good premise, lackluster execution.