Competence breeds confidence.  If you learn or improve upon a skill, you'll become more competent at it.  And, that competence can lead to increased confidence in that skill.  For example, if you are shy but you inch out of your comfort zone bit by bit, you will become less shy and more confident in your ability to talk to others and make meaningful connections.    Some books below are great resources to learn more about competence as it relates to confidence, while others are meant to help you increase your competence in a variety of skills.