Recommended PPC Reading

This is my short list of the most important books I've read in regard to Pay Per Click advertising. I've read a lot in copywriting and direct marketing, but these are the essentials, in order of importance.

ALL of these books can and should be read at least twice.

1. My Life in Advertising and Scientific Advertising by Claude Hopkins
2. Tested Advertising Methods by John Caples
3. 2,239 Tested Secrets For Direct Marketing Success : The Pros Tell You Their Time-Proven Secrets by Denny Hatch
4. Confessions of an Advertising Man by David Ogilvy

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Short, boring, to-the-point post :-)