Are you already highly productive, but want to be even more productive? You should read about systems and methodologies, and implement one rigorously.
Recommended: 4 Steps to the Epiphany by Steve Blank - breaks down the customer development lifecycle into actionable deliverables.
Do you feel like you're not being productive or shipping? You should read about procrastination and then diligently work to fix it, not spend more time studying GTD-type systems.
Recommended: Procrastination by Jane B. Burka and Lenora M. Yuen - the essential book on understanding and correcting your procrastination.