Caroline Cherryh is one of the foremost writers of ``hard SF'' working today. By this I mean that it is driven by speculation about the implications of technology on our society. This means no magic swords or other fantasy trappings.
Some of the series below are described as strongly connected. This means you shouldn't start reading the first volume unless you have them all on hand. Cherryh has a way of hanging you over some godawful nasty cliffs between volumes. In at least one case not only the fate of the protagonist but the fate of the species itself is in doubt---right at the end of a volume.
(By the way, Cherryh has done some good fantasy too, but I don't want to scare away any hard SF fans.)