alt Hacker NewsGot downvoted as expected, but I'm not trolling: book covers, just like the books themselves, are covered by copyright. I think the website looks really cool - but you cannot just use third-party material without permission. I know with AI companies crawling everything under the sun as training data, the idea of copyrighted material has kind of fallen out of fashion, but I think I've asked a valid question.
In the US, this use would be covered by the fair-use provisions of copyright law. You don't need permission to show a cover when writing a review of a book.