Ok, let me put it like this:
If you are playing a table-top game, and during an IC discussion between your character and the character of the other player, the other player suddenly pulls a sourcebook out of his backpack and has his character say to yours 'you are wrong, look, it says right here that I'm right' while pointing at the relevant page of the sourcebook.
I would think that is poor RP at best, and a horrible crossing of IC and OOC boundaries at worst.
Of course the sourcebook is 'right', insofar the gamemaster decided that is the common point of origin for his world. But using that literally as character knowledge, I can't imagine nobody having a problem with that.
I have no problem in the character using the knowledge from the sourcebook, as long as he referred to an IC source, a source which can be questioned to normal IC means. But I don't think a 'pulling the sourcebook' trick will do any kind of IC discussion much favor. What my approach is in such cases is send the relevant player a mail OOC informing him of said content and have the player decide if he wants anything to do with that content.
As said before, even with the same sourcebook in hands, one can still disagree about the meaning of the content, even if the content itself isn't in question.