Short answer: check the campaign's ACOS (Advertising Cost of Sale) — if the percentage you spend on ads per sale generated exceeds your per-book profit margin, the campaign is losing money, no matter how many clicks it generates.
The metric that matters most: ACOS
ACOS = (ad spend ÷ sales generated by that ad) × 100. If your margin per book is, say, 30% of the sale price, an ACOS above that number means you're paying more in ads than you're earning per copy sold because of them.
Other warning signs
Lots of impressions but very low clicks: the cover or title isn't generating interest.
Lots of clicks but few sales: the problem is likely in the listing (price, reviews, description), not the ad.
ACOS getting worse over time with no changes: may indicate automatic targeting started capturing less relevant searches.
Amazon Ads reports ACOS directly in each campaign's dashboard, with no need to calculate it manually, making it easy to check this metric regularly.Amazon Advertising dashboard, 2026