Rabbit Hole by Kate Brody

A twisty debut exploring the dark side of true crime fandom and the blurry lines of female friendship, perfect for fans of Gillian Flynn, My Favorite Murder, and Fleabag
Conspiracy theories from Reddit seduce a disaster-prone woman into an obsession with solving her older sister's cold-case disappearance
Ten years ago, Theodora "Teddy" Angstrom's older sister, Angie, went missing. Her case remains unsolved. Now Teddy's father, Mark, has killed himself. Unbeknownst to Mark's family, he had been active in a Reddit community fixated on Angie, and Teddy can't help but fall down the same rabbit hole.
Teddy's investigation quickly gets her in hot water with her gun-nut boyfriend, her long-lost half brother, and her colleagues at the prestigious high school where she teaches English. Further complicating matters is Teddy's growing obsession with Mickey, a charming amateur sleuth who is eerily keen on helping her solve the case.
Bewitched by Mickey, Teddy begins to lose her moral compass. As she struggles to reconcile new information with old memories, her erratic behavior reaches a fever pitch, but she won't stop until she finds Angie-or destroys herself in the process.
A biting critique of the internet's voyeurism, Rabbit Hole is an outrageous and heart-wrenching character study of a mind twisted by grief-and a page-turning mystery that's as addictive as a late-night Reddit binge.