I'm a freelancer in web dev and show biz and have done a lot of sales in that area. Brian Tracey's "Advanced Selling Strategies" is probably the best single book out there for individual one-on-one selling, especially if you also read "How to Win Friends and Influence People" 80% of reviewers on Amazon gave it 5 stars, and the rest 4. That's saying something for a business book! I would also highly recommend finding good books on negotiation too. Balancing relationship sales with negotiation and estimation is IMHO the hardest and most far-reaching aspect of being a freelance web dev. Iain Duncan
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