Dragon Wing is the first of the Death Gate Cycle series and it's by margaret wies & tracy hickman, the same people who did the other series I'm reading...again here is what it says on the back:
The Death Gate Cycle-
Ages ago, sorcerers of unmatched pwer sundered a world into four realms-sky, stone, fire, and water-then vanished. Over time, miagicians learned to work spells only in their own realms and forgot the others. Now only the few who have survived the Labyrinth and croosed the Death Gate know of the presence of all four realms-and evey they have yet to unravel the mysteries of their servered world...
Dragon Wing-
In Arianus. Relam of Sky, humans, elves, and dwarves battle for control of percious water-traversing a world of airborne islands on currents of elven magic and the back of mammoth dragons. But soon great magical forces will begin to rend the fabric of this delicate land. An Assassin will be hired to kill a royal prince-by the king himself. A dwarf will challenge the beliefgs of his people-and lead them in rebellion. and a sinster wizard will enact his palce to rule Arianus-a plan that may be felt far beyond the Realm of Sky and into the Death Gate itself.