The Difference Engine is an alternative history book with a difference (pardon a pun) - it is probably the first alternative history book I know that doesn't take a key moment of history to change. The book itself, although it starts normally, it ends up being strange and, actually, quite confusing.

The authors try awfully hard to hide the key changes in history, and if you started reading it without knowing in advance, you might be a little surprised. If you were ignorant of British political history and the history of computing, you might not even realise it was an alternative history book. I'm not too good on it myself, and had to look up many, many things for the purpose of this review. At least I learnt a lot reading it!

The background is that in this era, Babbage's difference engine has been noticed and adopted by industry in Britain and parts of Europe. The key turning point in history seems to be that Lord Byron became Prime Minister, and not Disareli, encouraging the Radicals to implement science and technology at their will. They win over the Luddite anti-technology movement, mostly through force, and establish the use of 'engines' as standard. Everything is run by engines, that are like large, overgrown, steam computers. Citizens are all given an individual number and their every move is recorded on a punch card somewhere.

It is against this that the story unfolds. The basic plot is that somebody has attempted and maybe succeeded to sabotage the Napolean Engine, the computing pride of France. A high-class prostitute gets entangled with a gentleman who promises to make her an adventurer. He has on him, he says, evidence of what has happened to the Napolean engine, even though he doesn't really understand it himself. It's all on a stack of white cards, encoded with some sort of program. Through a series of adventures the cards make their way to Lady Ada Byron (who was in real life a top-notch mathematician, if unrecognised in her time; the ADA computing language was named after her) who gives them to Edward Mallory, a paleantologist, for safe-keeping. Mallory had rescued Lady Byron from the clutches of a notorious Captain Swing, who in turn is now trying to get back the precious program cards through initimidation and general skull-duggery. Mallory is later approached by Oliphant, who is some sort of British secret agent (though no James Bond) and is offered protection. Mallory accepts reluctantly, as he finds himself increasingly out of depth.

The story does begin in this vein for the first two thirds of the book - an adventure, where the reluctant hero (Mallory) is thrust into action and seems out of his depth, until certain events force him to act decisively. It's very well written, and a joy to read, although I would recommend that you do background reading on the following before starting, because otherwise many things will seem confusing: Babbage, difference engines, Luddites, The Great Stink, Lord Byron, Lady Ada Byron, cold-blooded vs warm-blooded dinosaurs. To appreciate the secret of the program cards, you need to understand some mathematics, though saying exactly which areas will give away that part of the story.

But none of this will really help in the last quarter of the book. It becomes increasingly confusing and frustrating to follow. Perhaps its because I don't have enough background knowledge. But it's as if the authors lurch from one ending to another, not wanting to finish on an adventurer's cliche of all's well that ends well and getting entangled in their own confusion. It would have been enough to finish by explaining the secret of the cards, but to do it in this way... page after page of denuouement, I screamed at the book to get over with it already.

All in all, it is an interesting read up to a certain point, and I would recommend it to people who can appreciate the historical subtleties, but if messy endings are not your cup of tea, I would advise you to tear out and throw away the last twenty or so pages of the book before reading it. Trust me, it'll save you a lot of pain.