On Writing Well by William Zinsser (http://www.amazon.com/Writing-Well-30th-Anniversary-Edition/dp/0060891548 http://www.amazon.com/Writing-Well-30th-Anniversar

On Writing Well by William Zinsser (http://www.amazon.com/Writing-Well-30th-Anniversary-Edition/dp/0060891548 http://www.amazon.com/Writing-Well-30th-Anniversary-Edition/...) is fantastic. It's not specifically about ad copy, having passion about what you're writing, as well as the fundamentals in this book, will help a ton.