This is the second book in the series, but I haven't read the first and followed it well. You might get more from the friends' characters if you had read the first, but I would say it's not a necessity.
I thought the characters were really well developed, and you could really step into the world they are living in and feel immersed. The story itself is twisty and gritty, and I really felt for Harper as the story progressed. That being said, it was a little slow in places for me. If you enjoy a slower burn and building tension, then this one is definitely for you.
The final 3rd of the book really had me gripped, with the story being more fast-paced and all of the story lines coming together with a satisfying end.