The twin hearts of fantasy (and science-fiction, too) are deciphering the ideas that make the book work and exploring the worlds created by those ideas. In the beginning, all is mysterious--names and places and concepts are tossed around with little explanation--and gradually the pieces fit together and become clear. But a smart fantasist will always leave a whiff of mystery hanging in the air, a feeling that there are still frontiers to be explored (the better to set the stage for an infinite number of sequels, of course).
With Tolkien, I was always fascinated by the parts of the map that didn't enter into the storyline: Far Harad, the Northern Waste, Rhûn. As I got older, I also realized that a lot of fantasy never really dragged itself out from under Tolkien's shadow (trolls! swords! Old English diction!). But there are a lot of younger writers who are taking deliberate aim at the cliches of the past and I was thrilled to discover that some of them like maps too.
China Mieville's Perdido Street Station is a fantasy novel for urban planning majors. Instead of farmboys and fair maidens, Mieville gives us the teeming metropolis of New Crobuzon where five or six species coexist uneasily with a corrupt police state, organized crime, street gangs, vigilantes and all manner of religious cults, political movements, labor unions. Fittingly, the inner cover map delineates neighborhoods and provides a helpful overview of New Crobuzon's subway system.
To Mieville I would also add Neil Gaiman. Gaiman is famous for reworking ancient myths into a modern context, and in his first non-graphic novel Neverwhere he constructs a netherworld (London Below) out of the 40 odd abandoned Tube stations on the London Underground.
Ultimately, Neverwhere is not Gaiman's strongest book, but it is quite charming in how it weaves magic into the everyday of grit of London. Gaiman treads lightly -- I could easily imagine a series of books inhabiting this world, but many of his most promising ideas are given only a glance. In this sense, Neverwhere is a little sketchier and less ambitious than PSS -- but I thoroughly enjoyed its merging of fantasy and modernity.