Let's say you're writing a book in the fairly early days of urban fantasy--maybe 1996 or so. (That makes your book a contemporary of good old Anita Blake, or for the less literate, it's a few years after the Buffy movie but before the TV series.) You want magic in it but your repertoire to date is heavily science-fiction based, and you're not yet aware that vampires are apparently going to Run The Show for the next twenty years so you've decided to leave them out.
Walter Jon Williams was in this spot when he wrote Metropolitan. It's all about magic (though it doesn't use that word) so it's got to be called fantasy, and it takes place in a very human, very realistic urban environment. So I have to categorize it as urban fantasy, even though it's about as different from today's UF genre as you can get.
The work takes place hundreds (thousands?) of years in our future, when our cities have long since encircled the globe and run together, expanding and aging until the whole planet looks like downtown Hong Kong today. In the layers and layers of metal and concrete a power called plasm has been discovered: created implicitly by the energy humans invested in their creations, resonating with and amplified by spacial relationships between buildings and towers and bridges, plasm pools everywhere humans have lived and built and dreamed. It's pervasive, it's capable of creating life or destroying it, and it responds to human thought and will. But it's controlled: the Plasm Authority taps wells, diverts plasm to its own collectors and batteries, polices the sources and sells plasm to anyone who can pay--and that excludes almost everyone, including our protagonist Aiah.
The plot is intriguing and the work is full of fun characters, but curiously it's actually the background that is the most compelling part of this book. Remember the first time you saw Blade Runner: that dirty, lived-in city stuffed with teeming hordes of realistic nobodies? Metropolitan brings that same sense back, and holds it for the entire novel. From the cracked tiles and faded plastic chairs to the floating cars and extravagant glowing billboards, the world Williams creates is almost too tasty to let one concentrate on the action. Almost. And if you have the sequel (City on Fire), it manages to do the exact same thing all over again.
I loaned out my copy of this book years ago, and had to resort to back-alley deals to find another copy. Actually I got two this time, since I'm about to send away one of them to my sister and keep the other gem for myself. As I was re-reading it I kept catching paragraphs and thinking, "I should quote that in my review!"--but when I had reached a few dozen, I decided I would just recommend you read it yourself instead.