Scientific Advertising by Claude Hopkins
Published in 1923, this appears to be the first book on advertising to: a) apply A/B testing to prove campaigns, and b) focus on establishing feedback loops.
Read the last chapter first.
A rapid stream ran by the writer's boyhood home. The stream turned a wooden wheel and the wheel ran a mill. Under that primitive method, all but a fraction of the stream's potentiality went to waste. Then someone applied scientific methods to that stream - put in a turbine and dynamos. Now, with no more water, no more power, it runs a large manufacturing plant. We think of that stream when we see wasted advertising power. And we see it everywhere hundreds of examples. Enormous potentialities - millions of circulation - used to turn a mill-wheel. While others use that same power with manifold effect.
We see countless ads running year after year which we know to be unprofitable. Men spending five dollars to do what one dollar might do. Men getting back 30 per cent of their cost when they might get 150 per cent. And the facts could be easily proved.
We see wasted space, frivolity, clever conceits, entertainment. Costly pages filled with palaver which, if employed by salesman, would reflect on his sanity. But those ads are always unkeyed. The money is spent blindly, merely to satisfy some advertising whim.