Well written, with a sense of humour (and a surprisingly explicit sex-scene in the middle portion) the Difference Engine is set in an alternate "steampunk" universe (indeed it is credited with launching the genre) where the Victorian Aristocracy have been overthrown and the British Empire is ruled by savants, engineers and scientists. The story mainly follows the adventures of palaeonotologist Edward Mallory who has come into the possession (from Lady Ada Byron) of a mysterious box that is sought by anarchists as the basis for the overthrow of the Radical Meritocracy. However, his central part in the story is taken over by the shadowy Laurence Olifant towards the end of the book.
"The Difference Engine" makes much use of a number of story telling devices - there's a McGuffin (the box), a Chekov's Gun, a Femme-Fatale and a Man-of Action, and a twist at the end. The plot, however, is tenuous to say the least and is reliant on the McGuffin for driving the story along (well, that's what McGuffin's are for, isn't it?). I very nearly gave up on it early on because it seemed to me to be going nowhere. My persistence paid off, however, and it developed (slowly, I admit) to an unputdownable page turner.
It is certainly an enjoyable read but I must admit to having felt a little let down by the very end. What had I learned? Where had the story taken me? Where was the narrative linking the start, middle and end of the story? It is the lack of a strong plot that lets the book down and the strength of the story telling that lifts it.
I certainly recommend it but at the same time suggest that you may need to work hard to maintain interest in the early scenes. (Much) better than average but not exactly top notch...