Yes, using a page based website for your book makes sense. A book by its nature is linear: chapter one, two, three…
But writing a book? (which I have never done) It's a creative process. When I start a project, what I thought at the beginning is not the way it finishes because as I work I discover new ideas.
My brain bubbles around and can feel disorganized. I like the extra structure which you have with categories and tags. When I write something and tag it I can go back to that tag and see what else I've written on the same topic—or I might discover that I've ignored a topic by mistake or didn't consider at the beginning. I think of tags and categories as a very flexible outline, not so linear as the outline process we learned in school.
Of course, you need both linear thinking and bubble thoughts. Does that make sense?
I'm working on a page-based wp.org site for my job and my thought process began with weeks of experimenting with various themes and the menu structure. It's live now and I think our Board of Directors will like it. It was a creative process and I wish now that I'd made a private blog to use for keeping track of the dozens (hundreds) of elements which went into making it. Much cleaner than the piles of post-it notes on my desk!
So my answer about whether a page based site is better than a post based site is: both.