Colored Stone Essentials is the text that goes along with GIA's class of the same name. It covers most of the basics about colored stones that a person just starting in the industry needs to know, but if you've been in the industry for a while, you will probably have picked most of most of this already. I'd recommend this mostly for someone trying to get a head start on a career in jewelry or maybe for someone who wants to be a more informed consumer.
Great intro into the world of Colored Stones - gives you vast amounts of info, yet also makes you aware that there is so much more to learn about them. Which you can through GIA's Graduate Course for Colored Stones.