Getting Things Done (David Allen)

The granddaddy of modern productivity systems, published in 2001 and still the gold standard. GTD's core insight is deceptively simple: your brain is for having ideas, not holding them. The five-step workflow — capture, clarify, organize, reflect, engage — externalizes every commitment into a trusted system so your mind can stop the anxious loop of "what am I forgetting?" The two-minute rule alone (if it takes less than two minutes, do it now) has saved more collective hours than any app ever built. GTD's weakness is setup cost — the full implementation takes weeks — but its principles work even if you only adopt half of them.