[That third] page is definitely plugged in to the core of Moore's magical thinking, for sure. He has likened existence to a book before: the idea that past, present and future all exists, and the illusion of linear time is created by us moving through the book a page at a time ("a most timely gift!"). Due to our perceptions, we think the book is being written as we go, when in fact the entire book exists. And in Moore's Jerusalem, we see how some people's consciousness is able to move forward or backward in order to relive different parts of the "book" after their mortal existence ends.
Moore has also talked about the idea that while this notions suggests a deterministic outlook wherein we can't control what happens, our consciousness is completely free to *interpret* these happenings, in the same way that one can't change what one reads in a book, but one can interpret the content of the book in a million different ways ("it shall have meaning!")
For the potboiler purposes of Judgment Day, Moore creates an iteration of this book of existence where people can cross things out and rewrite them, and that of course goes against Moore's worldview ... but apart from that, this whole idea is really core to Moore's thinking. And that's not even getting into Mercury's "like me, it's made of language." Which gets back into what you and others have mentioned before, Moore's notion that the gods only exist in our minds, and our minds create thoughts in language. Therefore, the gods are "made of language," and books are also made of language, ergo ... books are like gods. Which also speaks to what Moore has said before about the idea that mortal existence (or the book of mortal existence, as the case may be) IS god, experienced at a different level of perception. (In qabbalistic terms, mortal existence being at level 10, and god being at level 1.)
Also, what you say about the macguffin being a book instead of a weapon is a fascinating point, and reminds me of an observation in Geoff Klock's book, "How to Read Superhero Comics and Why." In contrasting Moore with the other great comic-book maestro of the '80s, Frank Miller, Klock notes that Miller's metaphors and imagery are always violent, while Moore's are literary. Even a shockingly violent moment like the Joker shooting Barbara Gordon in the stomach and crippling her is described in literary terms, the Joker speaking of her as though she were a book. ("There's a hole in the jacket and the spine appears to be damaged ... but then, that's always a problem with softbacks. God, these literary discussions are so dry!")