Getting Started with Getting Things Done (GTD) by David Allen

From my perspective the goal of David Allen's Getting Things Done (GTD) is to get you focused on the things you want to do, when/where you can do them, and with the energy/motivation you might have at the moment of decision.

I cannot recommend this book Getting Things Done enough. 

If you are an audio learner then you can check out their free podcasts.  One of the best intros of GTD is by Meg Edwards.

There is also an awesome 2004 interview with David Allen at the Atlantic magazine.

Check out DavidCo.com for more resources.