One of the most valuable books I have ever read since becoming interested in Productivity has been Creating Flow with OmniFocus by Kourosh Dini. Not only was it a complete guide on how a master uses a tool like OmniFocus, it also touched a lot on the theory behind WHY we should adopt various productivity techniques.
It's thanks to this book that I was able to read another title from the same author - Workflow Mastery. Workflow Mastery is not a book tied to any particular task management application at all. Far from it, the words contained within are all about us, human beings and how we can manipulate our environments, our thoughts and build foundations for workflows that will lead to a greater return on our efforts.
It feels like it needs to have the word Zen contained within the title somewhere because the amount I learned on how to work with focus, make clear decisions, slay the dragon of procrastination and stop bouncing from one task to another, is truly staggering.
I can't recommend this book enough.