All of ye, go read the updated Encyclopdia entry on the Codex!
<http://myth.bungie.org/legends/encyclopedia/what.shtml#codex>
The Codex is an infinite book, but it is of mundane size and only weighs 15lbs. And you can't know whatever you want from it because when you open it, it opens randomly to a page about someone or something usually having little or nothing to do with you. Mazzarin had a knack for opening it to a page near his own entry, though.
As for fate and free will, it is possible to have free will and preordination at the same time. Ignoring the chaotic effects of probability, everything in the future is determined by things that happened before it, eventually being determined by things happening in the present, including the choices we make. But everything in the present, including our choices, is determined by things in the past. And since the past it already predetermined, so are the choices we make, and thus the future.
So we're free to make whatever choices we will, acting as we would of our own accord - but our accord is determined by what kind of person we are and the circumstances we're in, which are already determined. And thus, so is the future.
The monkey wrench in this simple statement is that the Codex tells the future, meaning that the present is determined by the past and a small part of the future as well. So, knowing the future, you can change it.
However, a couple of things make the Codex fit into the world without causing any paradoxes. The simplest one is the standard time-travel paradox-fixer: multiple timelines. So if you read about something in the future and then change that future, what you read about still happens, just in a different timeline where you didn't prevent it.
But that's a cop-out excuse, since things told in the Codex seem to be the future of the one and only true timeline. Yet, since there are gods and magic and fate and such in Myth, the Codex can still fit in with the other standard time-travel paradox-fixer: causality loops.
The fact that you read the Codex is factored in in what the Codex says to you. So if the Journal Writer hadn't read about the Summoner, maybe Soulblighter would never have known about him (the Baron had the Journal, and he could have told SB about the Summoner), and Myth II never would have happened - so the Codex wouldn't have told the Journal Writer about the Summoner. A causes B, B causes A, and so on.
The only problem with causality loops is how they come to be in the first place; things in the past and future and alternate timelines have to be set out and coordinated by someone or something outside normal space and time. But that's no problem, because in Myth, we have the gods, who can do whatever they want and make causality loops to their hearts' content (presuming they have hearts). They could subtly influence events happening after the info from the future arrives, making sure that they happen like the info said.
You know, this is kinda like wave/partical duality. For those who aren't into physics, a photon is the smallest unit of light. Note that I say "unit", not "particle". Because a photon doesn't always appear as a particle - sometimes it appears as a wave in space-time. Many other subatomic particles behave in a similar way. If you were to fire, say, an electron, toward two tiny slits in a lead sheet, with a piece of film behind them, you would get an image of two ripple patterns intersecting. That's because the wave of the electron passed through the two slits into two smaller waves, which intersected with eachother.
HOWEVER, if you were to set up sensors to see which slit the electron really went through, you would just get a dot on the film behind the slit that the electron went through. When observed, it appears as a particle in a specific place; when unobserved, it appears as a probability wave, not anywhere in particular, undetermined.
Now, with the Codex in Myth, it seems that reading about something in the Codex determines that that something will happen. If you don't read about it, it may or may not happen. But as soon as some info about the future arrives in the past, that future is destined to happen.
Interesting...
