O'Reilly -- Managing Digital Images: Applying Ratings and Keywords:

"The market value of a photograph is dependent on your ability to get that image into the hands of someone who wants it. All of the work that you do to rate and group your images will add value to them, by making them easier to find and bring to market. And don't forget, sometimes the "client" is you. By applying a sound rating system to your collection, you can get the maximum commercial, artistic, and personal value out of your images."

I'm still struggling with a good organisation system for my images - perhaps Aperture will be it, I don't quite know yet, since I'm too cheap to a) upgrade to OS X 10.4, and b) buy Aperture (a would haved to come before b, of course). I know I'll have to get there before I start trying to be a pro about these things, but still - it's just All Too Complex for me at the moment.

However, this book is on my wishlist - perhaps, one day, I'll organise all the ones I've already taken...