This is a very, very, very good book. Get your hands on it as soon as you can and read it. For best effect, read its predecessors, "Quicksilver" and "The Confusion", first. It is the third and (apparently) final volume in a series - the Baroque Cycle - and the overall impact builds up steadily over the three books.
The entire cycle (the author apparently doesn't like the term "trilogy") is set in the late 17th and early 18th centuries and views its characters though a number of themes - Natural Philosophy, war, money, commerce, alchemy, slavery, religion and many more. My impression was that in this volume, the themes go deeper, and Stephenson works harder on them, than in the preceding volumes. Despite this he succeeds in maintaining the pace, a trick which the earlier two (especially "Quicksilver") didn't always manage quite so well (though they were still excellent overall). It could be though that those earlier books did the hard work and set the scene.
Anyway, "System of the World" brings things to a tidy(ish) conclusion. There are suprises. There is a detective sub plot (along the lines of Samuel Pepys meets John Rebus). There is minute detail on London. (Please, someone, organise a Baroque Cycle walking tour - I'm sure it would be more rewarding than for certain bestselling historical novels I could name).
Actually this is the third in a series of four - the fourth, Cryptonomicon, which is set in the 20th century, was published first. The relationship with Cryptonomicon is loose - broadly the characters here are ancestors of those in the later (er, earlier) book and there is geekish fun to be had in watching Stephenson dispose everyone correctly by the end of "System". However many of the themes are the same, and in fact the ending of "Cryptonomicon", which I have seen some reviewers here criticise as just too implausible, fits better with the earlier volumes - where fortunes are gained and lost through treachery and chance - as background.
I do hope that Stephenson will follow up this story, in some way - I think I see hints in the text that he might: at least one character remains a real mystery and some of the themes are left open. Perhaps, though, for reasons of symmetry, that would have to be set in the far future.