When I analyze the ads of publishers who come to me for consulting, I always see the same issue: a lack of understanding of the metrics and how they are connected to one another. ACOS, CPC, and CR (conversion rate) are tightly linked, and they are highly specific to each individual book and even to each version (paperback or hardcover). There is only one “sweet spot,” and it must be found by considering the book’s price and the net royalties per copy sold. Not every bid is sustainable for every type of book, you can’t cheat the math.