Okay, so A Court of Splintered Harmony is the sixth ACOTAR book. The big bad is a death-god named Koschei the Deathless. He's been trapped for ages, but now he's stirring and gathering allies to attack the human lands.
The Night Court wants to stop him, but they're a mess. Everyone is still recovering from the last war. On top of that, recent betrayals have left the group divided and reeling.
So the whole story is about them trying to fix their broken bonds. They have to pull together and rebuild their old alliances. If they don't, they won't be strong enough to fight Koschei's rising shadow.
Yes, it is. A Court of Splintered Harmony is the official sixth book in the A Court of Thorns and Roses (ACOTAR) series by Sarah J. Maas. It's also subtitled "The Valkyrie Cycle, Movement I".
You don't need a complicated order to get there. Just read the series in the order they came out.
Here's the list:
That's the easiest way to follow the story. A Court of Splintered Harmony is the sixth book.
A quick heads-up, though. This book is part of a new story arc called "The Valkyrie Cycle." It picks up after the events of the Crescent City series, so reading those books first might help you catch all the connections. But if you just want to stick with the main ACOTAR books, the list above will get you where you need to go.