Releasing the hook grip was touched on on page 59.
However it goes into more detail during the description of the third pull on the bottom of page 96.
It states... "if the athlete chooses to release the hook grip in the overhead position, this must occur extremely rapidly and at the last possible moment, as the hand is finishing its final turnover from a neutral to extended wrist and the athlete begins driving up against the bar. Prior to this, the thumb and fingers will still be actively resisting the forces of the bar and the thumbs' release will cause the bar to slip."
Now keep in mind that there is no "you have to keep your hook grip". This is a personal choice. I keep my hook grip because it makes me feel more solid. Greg releases his hook grip. Play around with it and see which you prefer.
And yes, Greg does frequently say that releasing the hook grip should be taught in the very beginning, because it can be difficult to go back and start releasing it after getting used to holding it.