Introversion/Extroversion may determine leadership style, but by themselves they are not necessarily a predictor of successful leadership. Successful leadership is typically determined by a set of personality traits that can by combined to form competencies such as adaptability, communicating effectively, making decisions and driving for results. If you are sincerely interested in what are the predictors of successful leadership, then you should study I/O psychology instead of reading self-help books.

For example, you may be an introvert and still lack empathy for your colleagues and be completely unfit for leadership.