The White Road by Lynn Flewelling, 1st ed 2010.
The White Road picks up where Shadows Return left off, so you'll be extremely lost if you didn't read SR. You know all those questions left dangling all tantalizingly at the end of SR (and alluded to throughout the first 3 Nightrunner books), like "what on earth is a rhekaro?" and "why did the Hazadrielfaie fuck off to the mountains of the north, anyway?"? They're answered very satisfyingly, yet there are enough threads left dangling for further sequels: Phoria is still at war, and still hates the wizards of the Oreska, and distrusts Nysander's Watchers.
Alec and Seregil are dealing with the fallout of the Plenimaran plot that got Alec captured and resulted in Sebrahn's existence. They retreat to Aurenen for a while, where Seregil's nemesis Ulan of Viresse plots against them. They decide to go back to Plenimar to retrieve the book that Yhakobin used to create Sebrahn, to attempt to destroy it, or at least get it out of enemy hands.
Meanwhile, the Hazadrielfaie set out to destroy Alec and Sebrahn, because they exist and they should not.
I admit, I got really bored of all of Alec's "mothering" instincts toward Sebrahn. That whole subplot did nothing for me, even if Sebrahn himself started to grow on me. Aside from that, I loved the book. A lot of the scenes in Aurenen were very poignant, especially the scenes with Seregil and his family. Then the action started, and I didn't want to put it down.
I recommend this book if you've ever read Flewelling's other works and enjoyed them (even if you didn't like SR very much because of Sebrahn). If you haven't read her other works (and you like fantasy), go pick them up!