I would say non-book sources are allowed.
Personally, I would be inclined to allow all sources, but only the ones listed in the original post can be used as outright fact (and, even those, can be disputed, if we are capable of doing that with civility).
However, anything you see that helps make your point, you could mention, but that is not to say the the community is necessarily going to accept it as a valid source.
Point is, there is a ton of information out there, and not all of it agrees. We shouldn't arbitrarily reject certain sources. Instead, we should view each source with a critical eye.
Instead of going "OH MY GOD THAT IS FOX/MSNBC YOU CAN'T USE THAT"
We should go "Yes, I see your point, but that source is known to be biased. I found a source that disagrees." and then proceed with civil discourse from there.
But, I am just one of two moderators. This is not a final decision.