Highly recommended – “Don’t Just Roll the Dice“, a free e-book by Neil Davidson.
Although scientifically purer, it often doesn’t make sense to change a single variable at a time. Theoretically, you shouldn’t change the price of your product, your discounting strategy and the types of bundle that you sell, all at the same time. But practically, it can be the right thing to do. It’s more useful to fix the problem than to understand why it’s broken. When a scientist goes on a blind date that doesn’t work out then, in theory, he should fix one variable at a time, and re-run the date. First, he should change the partner but go to the same film and buy the same flowers. Next, he should keep the partner the same, vary the film and keep the flowers the same, and so forth. But the pragmatist in him will, or should, change the girl, the film, the flowers, and buy some new clothes and shave too. If it works, he might not understand why, but at least he’ll have a girlfriend.
Complement this with some reading on basic microeconomics.