Rich said: So I am a Northeast guy, and can't really answer this with a specific book or books. I do know from friends in Cali that Jepson is the botany bible there, so to speak. It is more of a professional text so it may be beyond where you're ready to go now, depending on where you're at with it. I don't know offhand for AZ or WA.
That said, for any region what you want to do is reach out to botany groups, botany professors, botanical gardens, etc. to see what they recommend as best for that region - again at varying levels depending on where you're at. For instance I visited the wonderful Phoenix Botanical Garden a couple of years ago. I'd start there and see what they recommend. But make sure they know you're a more serious student of botany and want the good stuff! You may have to ask around to get the right person to talk to. See what local folks are using to teach field botany in their area. Find a wildflower or botany group to go on walks with.
That is really the best way to find out the best books for a given area. Of course to some degree you can look this up online too.