This is not a traditional thriller or mystery, but a literary mystery. It explores the ripple effect of the death in the 2019 section through a variety of POVs and articles/online posts. (I have all the POVs listed in the spoiler section below). The 1999 section explores how the actions of Karlie and those in her life at the time built on each other to ultimately converge in her death. There are occasionally excerpts of Karlie's writing between chapters. 

To me, the 2019 section and the 1999 section felt like different books. There were POVs that could have been removed without affecting the story. This is partly because the 2019 section has a split focus - some chapters are focused on Karlie, but Joy's divorce and the effects on her family are given almost equal time. 2019 felt more like a collection of related short stories than a narrative to me.

Chapter POVs:

2019
Grand Mal - Joy
Unit 7C - KC
r/JusticeforKarlie - Joy
The Veldt - Sean
The Dark Forest - Sheri
The Weeper - Maggie
1999
Universal Love - Joy
The More Loving One - Jacob Hendrix
The Gathering - Joy
Bonedigger - Ian
The Walk-In - Lila
Karlie - Karlie
Dark - Kent
2019
When we were gone astray - KC