What people skills do you wish you learned earlier in your career?

What people skills do you wish you learned earlier in your career?
For the past several years I've been putting in a lot of time into learning and sharing people skills (without the bullshit). I'm interested in skills from dealing with your own emotions, through communication with close co-workers to high-stakes negotiating. I'm curious: what are some people skills that you wish you had learned earlier in your career or that you wish your co-workers had easier access to? Also: what are your favorite books and/or other resources that helped you?