Earth Made of Glass

In A Million Open Doors, John Barnes introduced the universe of the Thousand Cultures - in which, a thousand years hence, humanity's hundreds of settled worlds are finally coming back into regular contact with one another via the recently invented "springer," which affords instantaneous travel between worlds separated by light-years.
But reknitting mankind's diversity is a challenging task, requiring skill, ingenuity, and the patience of professional diplomats. Which is what Giraut and Margaret have become, twelve years after the events of A Million Open Doors. Now their task is to bring into the community of the Thousand Cultures the one human world that has yet to build a springer - the terrifyingly hostile world of Briand, a planet of boiling acid oceans whose only habitable portions are Greenland-sized sub-continents that project out of the heat of the planetary surface into its temperate stratosphere.
But Briand's physical hostility is nothing compared to the venom its two human cultures bear toward one another. Into this terrible world come Giraut and Margaret to try to do the right thing by the Cultures, by the inhabitants of Briand, and by one another.