You might pick one book and use it as a Bible. Or you might think that you have to know ALL the herbs and read ALL the books. Try to find a middle ground here. That way you won't be overwhelmed. First, not every book will say the same thing about the same herb.
Opinions vary from author to author, region to region, and experience to experience.
For example, almost every book recommends Valerian Root for anxiety. Instead of calming down, they'll feel like they've been supercharged. I have 16 herb books on my shelf and only two mentioned this fact. No, it means that experience and knowledge varies from person to person. Don't read just one book.