After the Eclipse by Fran Doricott

Even though sixteen years have passed, Cassie has never gotten over the disappearance of her younger sister. Olive ran off while they were watching a solar eclipse in their grandmother's hometown and was never seen or heard from again. Now an investigative journalist, Cassie returns to the small town to care for her Alzheimer's-stricken grandmother. But another eclipse is coming up, and another girl has disappeared. Cassie knows in her heart that the two must be connected, but will anyone believe her when the evidence doesn't seem to agree? And will this finally be her chance to get answers-- and justice-- for what happened?

This is an intense mystery with a ton of emotional depth. The main character Cassie struggles with her grandmother's mental deterioration, a hiatus from her job, reminders of her sister's disappearance, and even reconnecting with her childhood crush, who happens to be a police officer in the small town. The eclipse looms over everything like a ticking clock, and the ending has lots of twists and turns that keep you on your toes. Highly recommended for fans of dark, thrilling mysteries like Tana French or Ruth Ware.