Here are two great watercolor books for beginner to intermediate level painters, both by artist Charles Reid. If you want to try your hand at portrait painting, then I recommend the first book. The other book is not specific to genre; it deals more with composition and simplifying your paintings. There are wonderful exercises that take you step-by-step through the painting process.
I've heard the process of learning described like this: "First, you don't know what you don't know. Then you know what you don't know. Then you know what you know. Finally, you don't know what you know (ie. you become so proficient in what you do that you no longer have to think about it). These books remind me that I'm at the "I know what I don't know" level when it comes to portrait painting. There is just so much to absorb, understand and practice.