Recently, author Andy Sernovitz asked me, "What do you think about my book cover?" We had a lot of other books around, so we started to analyze it in terms of those covers - how his worked compared to some of those, and how it could've been improved. Then we did the same thing for many of the other books. Prior to this conversation, I watched a lot of people walk up to this same book table and scan the books with their eyes, some never even picking any up. It's almost as if they simply judged everything by how it looked, and moved on.
Chip and Dan Heath's Made to Stick was the one most picked up because of the embossed strip of duct tape across the front. People wanted to feel it. The point I'm making is that the cover of your book has a big impact on who picks your book up. In the retail world, if no one stops and picks your book up, they're certainly not going to buy it.
Even if your book is sold online, how often do you see it (or anything for that matter) that doesn't have an image next it. Regardless of how digitized things become, image is important. Design communicates many other things that the title and author's name do not. Design might say what the tone of the book is, how serious the book is, and it might even visually support the theme of the book (Made to Stick is again a perfect example).
You've written a book on lean manufacturing, or employee retention, or organizational behavior, or leadership. You're an expert on the topic, and that means that it's quite unlikely that you also happen to be a practicing and knowledgeable designer. To remedy this, with a quick web search, you find a ton of companies offering cheap and easy cover design "from your supplied MS Word document." There's the first red flag NOT to take this route. Want proof? Ask any professional designer if they consider Word a design program and something terrible will happen to you. I guarantee it. But seriously, your cover design should be handled by someone using the proper tools with knowledge of what sells in the book market - particularly for the type of book you've written.
Now, this doesn't imply that spending a lot of money on a designer guarantees a great book cover. The point is to take this aspect of your book very seriously. Trust the people you put in charge of this. Look at their history of results. Choose someone whose work you admire - stacked up against the competition. Like Andy Sernovitz and I discussed his cover - you should have an informed opinion about what you like and what you don't.
The New York Times picked a couple covers that stood out in 2007, and posted about them at their blog. Following it is a list of opinions from readers. Looking at this as a case study, it becomes clear how strongly the public will react to what your book looks like. That is, if they even notice it.