Asking for 'The Pact' always requires a bit of clarification because I think there are a few novels with that name floating around. The one I'm most familiar with is by Jodi Picoult. It centers on the suicide of a teenage boy, Chris Harte, and the subsequent fallout for his girlfriend, Emily Gold, who survived the initial pact. It's a really intense family drama disguised as a mystery—was it a murder-suicide pact gone wrong, or was Emily actually trying to kill herself and Chris tried to stop her? The plot isn't a whodunit in the traditional sense; it's more a 'what exactly happened and why.' It digs deep into the pressure cooker environment of their intertwined families, their perfect-seeming suburban lives, and the terrifying, ambiguous line between love and obsession.
What I found most haunting wasn't the courtroom scenes, but the way Picoult unravels the kids' history. You see the childhood friendship, the parental expectations, the slow creep of depression that everyone misses. The 'main plot' is the investigation into the pact itself, but the real story is about how well we can ever truly know another person, even our own child.