In the image above, you see the reality of physical book sales. Most often, you don't get the benefit of your book being face-out on the shelf. Most of the time, your readers are going to first engage your book based on the spine.
Depressing? A little, especially if you've got a fantastic cover and a sorry spine. Sadly, I see sorry spines all the time.
So, what makes a good spine? Three things:
If you can't read the text on the spine of your book, who is it serving? It doesn't matter how much you love that typeface, if it's illegible, it's worse than useless: it's confusing. Fix it. Choose to serve your readers through clarity.
This gets particularly difficult if the spine of your book is tiny. Thin spines are really hard to put text on it. Don't try. You'll be glad you didn't.
Contrast, brightness, big, readable type. All of these things help your spine to make an impact across the room. This really dovetails into the clarity point — if it's easy to read, it's good. If it's hard to read, it's hard to sell.
I know you were waiting for this one. Creativity matters, even on the spine. Maybe especially on the spine. But, creativity is best when it stays within wise limits. So, a creative integration of the front cover treatment is great. If your background can wrap around attractively, it doesn't just help the visual, it also helps minimize the visibility of variances within the printer's work. Incorporate a logo that represents your brand.
Bottom line: get creative, but remember that clarity and visibility are more important than creativity!