Suspenseful and creepy, but parts felt like a sequel?

The prologue really grabbed my attention. It has a creepy set-up and I was really curious about what had happened and where the story would go, but when the narration shifted in the first chapter I was confused by the events. That's when I realized that this is a sequel (or at least a companion) to another book. When the narration shifted again, I was drawn back into the intriguing mystery. It ends on a very creepy and suspenseful note, but I'm not sure whether I would read the rest! Maybe I need to read the author's other book first!