We judge books by their covers, the proverb notwithstanding (love that word). At least we do so at first blush and in bookstores — which is why they get displayed in shops lying flat. This is a hopelessly irrational thing, as I know from having written a book or two myself: the cover design need bear no relation, either in content or in quality, to what's inside. Still… the wishful thought is that a good cover betokens a publisher's care which will also have gone into the selection of a good manuscript.
What makes a good book cover? Might as well ask what makes good design: it's a proper question with an answer too vexed for a fillip — or me. I'm like you, probably: like Potter Stewart, I find it hard to define but I know it when I see it.
To see the range of what is now — and has been — good design in book covers, go to the Book Cover Archive, where over a thousand images are available for your perusal. You can browse them by genres, authors, dates, designers, and a bunch more facets. And you'll find there a dozen or so links to further sites on book cover design.
six decades of Pelicans
…one of which is to the Pelican Project by Things Magazine (an eclectic site, by the way, on architecture and design that I can't praise too much. Pelican was a subsidiary of Penguin and published serious books cheaply between 1937 and 1990. The Pelican Project shows you the covers of most of their books, sorted by decade, so you can see the sometimes subtle change in design even for this most plain of publications.
Covers have become a whole lot… sexier since then. For some of the best take a look at the New Yorker's blog Well Covered. My current favourite discussed there is the one you see to your left. Imagine the persuading it took to take the author's name off the front. Compelling simplicity. (The author is Todd Kashdan, by the way.)
And check out the blog The Book Design Review for some lovely covers and, because a cover isn't everything — in book design, I mean — some good discussion of typography and layout.
[hat tip to Slawyer Michel-Adrien for his pointer to this topic]