Kevin Hartnett's debut book chronicles how Leo de Moura's Lean program, initially designed to verify computer code safety at Microsoft, has become a transformative tool for mathematically formalizing proofs and training AI systems. Tech companies like Google DeepMind and Meta AI now use Lean to reduce AI hallucinations and improve accuracy, culminating in DeepMind's AlphaProof achieving silver-medal-level performance at the 2024 International Math Olympiad.