Though technically a Moist Von Lipwig book, this could easily be thought of as an ensemble cast. Moist is indeed the central character, and the thread of the story follows him for the most part, but other characters share significant page time. Namely Commander Vimes, Sir Harry King, and a new character, Dick Simnel. 
 Like many of the more recent Discworld novels, Raising Steam chronicles another aspect of modernisation within Ankh Morpork. In this case, the advent of the steam engine and railway. Moist finds (or, perhaps more accurately, insinuates) himself into the growing venture. The story takes place against the backdrop of a world coming to terms with goblins now being considered an equal race, and the increasingly violent acts of the grags-- a faction of dwarves that believe in the old ways, and are willing to kill to protect them. 
 In fact, the book almost reads like a two parter, with the invention and growth of the railway against the backdrop of goblin liberation in the first half, and the perilous journey of the steam train against the backdrop of terrorist dwarves in the second. 
 All in all, it is a very nostalgic novel. Many other Discworld regulars make an appearance in this one, and there are many references events from other stories in the series. Objectively, I would say it is far from the best in the series, though that is far from condemnation in such company. Not speaking objectively, however, it is remarkable that Sir Terry Pratchett is still writing novels of this quality (and frequency) given his Alzheimer's, which (to judge by a recent TV interview) would seem to be worsening. 
 If you're a fan of the series, this is a must read, but you probably didn't need telling that. If you're new to Discworld, this isn't the best book to begin with, if only for all the references, which are explained, but will mean little to you if you haven't read the other books. If you've tried Discworld and were unimpressed, this is will likely not change your opinion.