We were going to write an article about RGB vs. CMYK color modes, but there are already some great ones out there, so we are just going to link to them.
If you are submitting images for our upcoming book, it's important that you understand what CMYK mode is. There will be color differences from what you see on your monitor to what you see in print. Understanding CMYK will help you compensate for some of these color changes and make your images look better on the printed page.