Depends on what you want it for. Best easy book for learning elementary gemology is Matlins (Gem Identification Made Easy now in it's 6th edition). Another good text is Peter Read's Gemmology, which is used as the text for the FGA exam in the UK. For exhaustive coverage of gems and various procedures, Gems, in its 6th edition, edited by Michael O'Donoghue, is great. Remember that there are several sites on line now with gemology courses you can take, too. I find it's nice to have a physical book to browse through, but I'm old fashioned.
If you end up wanting a complete set of inclusion photos, you'll want all three volumes of Gubelin and Koivula's set, but that would be pretty pricey...maybe $800 or $1000 altogether...and most of the needed inclusion photos are probably now somewhere on the internet.
Once you know a little about gemology, I think the best book is Hanneman's Affordable Gemology, which will teach you how to identify stones with little or no equipment, if you're clever enough and practice enough.