I think Google Books is helping with this a bit. I have been able to find specific passages in books to link to on a couple of occasions.
Basically, I think sourcing needs to be readily available to your opponent. If it isn't available to them without a fee they may not be able to pay, or an unfeasibly long assessment time, I don't think it's within the spirit of fair debate in the context of the casual internet.
It's just not practical for this medium of debate. Hell, that wouldn't even be acceptable in formal in-person debate. You're expected to provide your sourcing during the debate.
There are several things, especially historical points, where I remember the point from reading it 20+ years ago, but couldn't for the life of me remember where I read it.
If, when defending your support for Donald Trump, and your response is,
"But but but... HILLARY!!!", then you lost the argument before you even began.
There are others that are less intense and where suggesting a book for someone's interest in learning more about a topic is a useless suggestion to someone hoping to further their knowledge.
Context cues, I suppose.
"A democracy cannot exist as a permanent form of government. It can only exist until the majority discovers it can vote itself largess out of the public treasury." Attributed to Alexander Tytler
A fanatic is one who can't change his mind and won't change the subject.
A lie gets halfway around the world before the truth has a chance to get its pants on.
I read an awesome article a few years about about how cutting back on privileges and early release in California hurt the recidivism rate as well as causing prisoners to act worse than they had (if there is no incentive for good behavior and attending classes - why do it?)
Damned if I can find it again. I read it in print, and my online searches, when I look for it, don't find it.
But I still use what I learned from that article in discussions re prisons/incarceration rates on forums. People can choose to believe me or not.