I just pick up my PHB if in doubt. But yeah, for newer players it might be a good idea. On the other hand it could be a bit intimidating for some, so I understand why Larian might not want to. An encyclopedia, or menu accesible from the main menu, might not be a bad shout though, post release.
Since Larian made changes to some classes the PHB won't align perfectly with what we'll get in the game, and that can lead to situations where you've carefully planned a build that doesn't translate well to the game, or cases where the game actually makes a specific class better or more interesting than it normally would be, but you wouldn't even consider if following what you know from the PHB.
I have seen button “details” during the character creation. I would assume that it will contain information regarding class progression.
I make the same assumption, and I think it should be available both in character creation and when leveling up.
Screenshot from CC to put the details button in context: