GTD introduces the concepts and prescribes behaviors to create a mind like water.
GTD Fast has editorializing that you don't have in the audiobook, and I found many golden goodies there.
MIAW presents GTD as part of a greater whole, and understanding of which permits fine tuning and custom-tailoring of GTD to achieve mind like water/state of balance more often.
My experience is that the essential concepts are in all of David Allen's works, with differing emphases... which may speak to an individual better at different times. David Allen must feel like the preacher whose congregation complained that they had heard all of this before... to which he said "and you will hear it again and again, until the same old sinners stop committing the same old sins."