Roofworld by Christopher Fowler

You won't see them unless you know where to look...
High above London's teeming streets exists a timeless universe with laws and codes known only to itself. Suspended in the skies by a complex system of cables and wires, two rival factions prepare to do battle for control of their world and the eventual manipulation of the city below...
Welcome to Roofworld.
Rose, a beautiful, feisty, amateur photographer - and Robert, a shy but cynical scriptwriter, witness a rooftop kidnapping by chance. And then they hear of strange rooftop murders that are being discovered almost daily - and they want to know more. But in their clumsy efforts to understand, they become caught up in an intense power struggle between honest seekers of another life and the consummate evil of a power-mad leader in control of society's hopeless. Together, Rose and Robert become inextricably caught up in a violent war among a nearly invisible people who could easily control the future of the world . . .