I don't like the idea of making books off-limits to players. Also, I don't care at all about this overly-vague "sense of wonder" idea, and I certainly don't think that WotC should build the rules to encourage that playstyle. I really don't see why player ignorance is something to be idealized... I never even had a "sense of wonder" in any D&D game, but I have had no problem enjoying the game and various campaigns very much.
Anyways, as a player, I like knowing things. Tracing back DM plots to brief lines in some Eberron sourcebook is fun. Knowing that Kaius, the King of Karnath, is a LE vampire as a player, but dealing with the fact that as far as the PCs are concerned he is the greatest king in the world, is far more dramatic and interesting to me than some cheap "surprise! He was a vampire all along" bait and switch.
Similarly, the fact that I read the stats for the Tarrasque when I first started playing the game would make me more terrified of fighting the thing than any DM's description could ever be. Heck, if for some reason I didn't read the statblock and the DM sent it at me when I was level 9 or such, I would probably just buckle down and try to fight it, no matter how the Dm prodded me, simply because without any kind of foreknowledge it is just another monster. The sentiment that "oh **** it's the Tarrasque" is 100% metagame knowledge, but it is far more real for the players than the PC's in-character reactions, and it is a lot of fun.
If I am to draw an analogy here... Quite often, in movies, books, and TV shows, the perspective of the story will shift completely away from the heroes, and show the actions of the villains or other characters. This gives the viewer more information than the main characters, which lets the viewer come to conclusions more quickly than the characters can, and helps add complexity and dramatic tension. Often, this improves the story quite a bit, and I think many stories could serve to use this technique more, rather than less.
Because the players are the audience just as much as the actors, there is quite a bit of validity in giving them more information than the PC would have access to, simply because it adds the same tension as it does in movies. By definition, this is metagame knowledge, but this just shows that metagame knowledge is not bad in of itself, if your objective is for the players to have fun and be immersed in the story, rather than just have them be characters under your control.