It depends.
Are you interested in selling books? If so, then where will they sell? Bookstores won’t stock books that don’t have a good spine and a commercially viable cover. And on-line stores like Amazon use thumbnails of the cover to help sell books because good ones work.
Without a good cover, your readers won’t look at your book long enough to get hooked. There have been a few exceptions, as with any rule, but not many. And who wants to make it any harder for your book to succeed in this world?
So packaging matters, even when you’re selling words. Allot enough resources to guarantee a good cover if you want good sales.
Now for the hard part: what makes a good cover? I have a few thoughts, which may be all wrong, so please feel free to contradict or argue with me in the comments:
Spine: Should be clearly legible from a distance of 3 to 4 feet, by your target audience. (For example, if they’re boomers, their eyes may no longer be good, so make it even larger font, and less ornamental.) If it’s popular non-fiction or fiction, consider including a thumbnail image.
Front cover: Look at the books on the store shelf where you want yours to appear. What do the successful ones have in common? Your design should have a similar design sensibility (color schemes, type of art or lack of same, font type, overall “feel” . . .), and similar amounts of information.
Back cover: The copy here (or on the jacket flaps) is critical. You have a very few sentences to tell the reader why they’ll be glad they bought your book and not the others that meet their needs. You may want to use blurbs (do other books on the shelf?) or summaries of benefits (not features) or plot synopses. Whatever you say, make it count!
Really, I’m not the person to ask, so I’m opening it up to all of you. What grabs you and makes you pull down a book (other than a known author or branded series)?