Most impactful tool for your industry?

So far my most useful tool has been my plumbing code book. It's honestly been what has guided me to start my own business 3 months ago. I'm hoping that as long as I follow my code book I'll eventually get to be more successful. Right now my biggest problem is I don't have enough customers because I'm kind of in an economically depressed area but I'm within an hour of all four directions of access to tons more customers.