Mother of Winter by Barbara Hambly (The Darwath Series)
This book takes place 5 years after the Darwath Trilogy. The Dark has left the land and everything seems great and the people are working on rebuilding their lives and their world except something isn't quite right.
Something weird is happening and no one is quite sure what it is even. There is a new plant that no one has ever seen before and it was never mentioned in any historical writings or the verbal histories of the nomadic tribes. Other things are happening too but no one is quite sure what it is, why it is and where it is coming from.
This book was creepier than the first three (or maybe I cared more about the characters at this point). I had to stop halfway through and read a couple of other books before I could finish it.
There is one more in the series and I'm looking forward to reading it - later.