Well, murders take place in both types of chapters of the story. Someone's trying to solve them, but... That's not really what makes it all that mysterious, I guess. Since half of the chapters are fiction, at times, the people who were killed in one chapter would be alive in the other one. It's easy to follow at first, but after a while the realistic and fictional aspects get mixed together, and it makes it harder to keep track of what's actually happening and what's just made up. It makes a vague borderline between the two of them.

[And somehow it had made Riki feel like something was up in his own life that demonstrated a reality versus fiction theme, but...]