Getting Things Done (David Allen)

The granddaddy of modern productivity systems, published in 2001 and still the reference point. GTD's core insight is deceptively simple: your mind 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 (if it takes less than two minutes, do it now) is the piece most people keep even after they abandon the rest. GTD's weakness is setup cost - the full implementation takes weeks - but its principles work even if you only adopt half of them.