Zahn, Timothy. Icarus saga.

"A Baen Books original." -- Title page verso

"For years Gregory Roarke and his Kadolian partner Selene worked as crocketts, combing through the atmospheres of uninhabited worlds for places that might be colonized or hold valuable resources. Now, they work for the Icarus Group, a top-secret government organization hunting for portals created by a long-vanished alien race, portals that can teleport a person hundreds or thousands of light-years in the blink of an eye. Usually, those hunts are long and tedious. But Roarke has now been handed an intriguing offer. A criminal boss, Robertine Cherno, will hand over a hitherto unknown portal to the Icarus Group in exchange for Roarke and Selene agreeing to transport a passenger named Nikki across the Spiral. There's only one catch. Nikki is a professional, high-priced, highly feared assassin. And she's on the job. That would have been bad enough. But when the alien Patth also move to gain possession of the portal, bad quickly promises to go to worse. Especially when it becomes clear that Nikki herself is being hunted by someone."--