What books and strategies do you recommend?
Aside from the David Allen suite, I've been reading The Checklist Manifesto and its changing the way I look at my to do and context lists. I also read about Steven Covey's 4 quadrants (importance and urgency grid) and looking how I can apply it to GTD. Have you read any books / heard of any strategies that you use to supplement GTD?
Lots of additional reading
This is an excellent question and one that I'm often thinking about and exploring.
I listened to Wild Success in 2013 webinar, during which I heard DA mention a book that deeply influenced his views on visioning called Release Your Brakes! I found a used copy and have been digging into it.
I also listen to as much GTD Connect audio as possible - from these I've found many interesting and useful resources or new ways of thinking about something. Have you listened to the recent interview DA did with Dan Pink? I must have written a full page of notes from that.