It depends on which humanity, of course, and on the specific topics of the paper. Like Ghavrel, I seriously doubt writing a paper based exclusively on Wikipedia will get you very far in any serious class, but on some subjects you can probably find everything you need online if you know where to look. Particularly when the topic of the paper is something quite recent - contemporary politics, history of the past fifty years, that kind of thing - *and* related to the Anglo-Saxon world or other countries that are wealthy and well-represented on the internet. For the rest, Wikipedia and other reference sites will usually give you something to start with - and sometimes even name the very books you need to hunt down - but even a student who doesn't want to do much effort will find himself forced to hit the library.

There is no reason why work cannot be published on the internet, but that will take enormous efforts. Even if all new academic books were published on the internet as well, we'd still have centuries' worth of books, magazines, newspapers, manuscripts and other documents that needed to be put online. And considering the costs of all that, it likely wouldn't be for free, so that a library which currently possesses a physical copy of a certain book would probably have to pay to access that same book online - that's doable for small amounts of books, but not so much for tens of thousands.

In short, if we ever get to the point books are obsolete in the academic world, safe to say that moment is still several decades in the future.

Fair enough.

Well, the purchasing costs make up the vast majority of that, and those are sunk. Selling them would yield a rather minimal compensation, considering how much being second-hand lowers the price of a book - frankly second-hand books tend to sell for much less than they're intrinsically worth. But yes, as I've said several times, I don't think HSes really need libraries now, if they ever did, so I won't get too alarmed until I see similar articles appearing about universities...