This is one of the first Steampunk books to show up, once the term 'steampunk' was in our literary consciences.
William Gibson and Bruce Sterling are two literary powerhouses, and their incredible skill shows in this masterful telling. Gibson is largely credited with being the father of 'cyberpunk'(it is worth noting that steampunk was initially a tongue-in-cheek reaction to cyberpunk), and Sterling himself helped to define both genres.
In the Difference Engine, Gibson and Sterling postulate an alternate Victorian England where Charles Babbage's titular invention actually works, and ushers in an early age of steam and computers. This book follows three main story-lines, including the infamous Ned Mallory, who discovered the Brontosaurus.
The language tends to be a bit heavy, and that is perhaps my only complaint, as I love the imagery and alternate setting the two authors created.
This book was better read early on in steampunk's history, as much of its entertainment value lies in creating an alternate history(this was one of the first books to do so) and in showcasing the advanced steam technology. Once those things were done, they lost their value, and successive writers have had to learn new tricks to entertain the reader.
Difference Engine is still worth reading, even if you are already familiar with the genre, though you should perhaps be interested in heavy language like Charles Dickens, or you might find yourself working to follow the story.