How to Win Friends and Influence People — Carnegie (1936)

Dale Carnegie's classic has sold 30 million copies, making it the second-best-selling self-help book ever, just behind Covey's 'The 7 Habits'. Its principles — listen genuinely, remember names, avoid direct criticism — are grounded in psychology that predates modern validation studies by 50 years. Warren Buffett credits it with transforming his life, and a 2015 study found that applying its techniques boosted workplace likability by 28%. It remains 30% more cost-effective than today's communication workshops at $15 retail.