Is it possible to alter the way the Gamebook mode works? And if so how would I. I want to be able to have a sort of Codex for the player. So they can check out the relationship they have with characters or their stats et cetera. But I don't know how I would even begin to do this.

You can do all sorts with the UI in Quest:
viewtopic.php?f=18&t=5111

If you look down a few posts you will come to "How to add a status box", which is more-or-less what you want.

However, I am not sure if that it true of gamebooks, which are much more restricted. If I was doing it, I would make a text adventure and disguise it to look like a game book.
viewtopic.php?f=18&t=4772&hilit=gamebook