Dr. Robert C. Worstell, Claude C. Hopkins (2014). “Scientific Advertising - Masters of Marketing Secrets: From the First Great Copywriter”, p.51, Lulu.com
Dr. Robert C. Worstell, Claude C. Hopkins (2014). “Scientific Advertising - Masters of Marketing Secrets: From the First Great Copywriter”, p.25, Lulu.com
Dr. Robert C. Worstell, Claude C. Hopkins (2014). “Scientific Advertising - Masters of Marketing Secrets: From the First Great Copywriter”, p.40, Lulu.com