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"Meditations" by Marcus Aurelius (several translations, worth comparing for even more insight) -- this is a man who understood duty and getting things done!

When more sophisticated or trendy approaches to effectiveness reach their limits, you can't go wrong falling back on "Begin with the end in mind", "Sharpen the saw", classifying activities into the Four Quadrants, etc.

"Flow" by Mihály Csíkszentmihályi. Being fully immersed and engaged in an activity.