ibeforem
The Turn is essentially the story of Trent's parents, Kal and Trisk, and the role they played in The Turn -- where tomatoes became deadly to humans and the Interlanders begin to reveal themselves to humankind. Since Trisk is referenced a lot in the newest book, I was glad to read this and get a better idea of who she was and what she was capable of. We also get a younger version of Quen, which is a treat.
This is a worthwhile prequel, but you should read most of the Hollows series before you jump back to it -- I think you will have a better understanding of the characters and politics that way.