I'll defend your choice to go by cover here, from the perspective of someone in marketing.
The cover is the first contact point anybody has with a book. It is there to let the prospective reader know things like genre, style, intended target audience, etc. A good cover should match the contents to your tastes. A bad cover doesn't.
We form a large amount of our opinions on things from quite literally the first glance: people, food, vacuum cleaners, and yes, books. If the publisher has even a half-decent marketer on staff, that book cover should absolutely be intended for you to judge the contents before you first crack the book open.