Hear, Hear offers an interview with Paul Buckley, art director with Penguin in the US.
Book design has become one of the trendiest areas in graphics of late, perhaps in part as a result of a decline in the importance of design for popular music in the downloading era, but largely as a result of the death of the Net Book Agreement which, for both better and worse has irrevocably changed the book trade in the UK.
No matter what you're selling - a product, an idea or a skill - the presentation is just as important as the thing itself. Your customers will only do so much research (if at all) when purchasing your product, and when presented with similar choices, they will choose the one they feel most comfortable with. And that decision is most likely based on the packaging. Nothing illustrates this better than the experience of shopping for new books: before we even bother to read the description on the back of a book we have never heard of, we need to first notice the book and have enough desire to pick it up. And that decision is based on the book's cover.
Or, as Oscar Wilde said: "It is only shallow people who do not judge by appearances."
The interview is certainly worth a read, particularly the part where Bukley discusses illustration:
Some of the best designers I know cannot draw a stick figure, and I’d kill to have half of their design talent. That said, on the opposite side of that coin, many folks who both illustrate and design their own work, do some of the most unique work out there – as long as they don't create a style that becomes too rigidly signature and same ole [sic] and stale.