People should check out Google Books.
It lists current in-print books, often includes free samples, and has links to places to buy them. It's kind of like a slightly less user-friendly Amazon.
But... here's the cool part...
Google Books also has countless, tens, even hundreds of thousands of titles for free download. These seem to mostly be old out-of-copyright academic titles, typically from before World War I.
In some subjects like the sciences, books that out of date might not be all that useful. Having said that, there's a treasure trove in the history of science. There's the whole 19'th century literature on the reception of Darwin's theories, pro and con, for example.
But where this thing really shines is in subjects like Greek philosophy. Basically the whole canon of surviving Greek philosophical texts was complete by the end of the Renaissance. So historians of philosophy have had the exact same texts to work with ever since. All that's really changed is context, academic fashion and styles of interpretation. The 19'th century literature on Aristotle or Plotinus is probably just as valuable and just as interesting as anything that's being published today, and much of it is simply being ignored.
I've been poking around what's available in Asian philosophy and have been impressed. In the late 19'th century many Sanskrit and Pali texts were just being translated and scholarly interest was very textual as a result, with lots of commentaries and minute exegesis. Today we are starting to see more 'critical' and 'post-modern' interpretations. Personally, I think that much of the earlier work is more valuable than some of what's being produced today.
There's lots and lots of history, religious studies and Christian theology. (Theologians might love this.) The old Cambridge histories are there, the various volumes of the Cambridge Medieval History and so on. The Schaff-Herzog Encyclopedia of Religious Knowledge, as well as Schaff's multivolume history of Christianity. More cool stuff than anyone could possibly read in a lifetime. The fall of Rome generates an absolute flood of books. I even found a translation of the wonderfully-named 'Periplus of the Erythraean Sea', an ancient account (by a very literate Greek sailor named Nearchus) of sailing the trade routes on the Indian Ocean between India, East Africa and the Roman Empire.
Go to the Google search homepage. On the menu bar at the top, click 'more'. Then click 'books' on the drop-down menu. Or what the hell, just go directly there:
http://books.google.com/
A search page comes up where you can do a subject search on whatever's of interest to you. A new page of book results comes up. Then click 'free Google books' on the left hand side of that page.
That will give you a list of free books conforming to your search terms.
You can read them right there on the screen, off Google's servers. To download copies for yourself, click on 'download' on the upper right, or if you don't see it, click on the gear-shaped thing. You'll be asked if you want pdf or e-pub format. Then download whatever you want to your computer. I store them on a usb drive and usually squirt them over to my Kindle and Kobo e-readers to read them on the go. (I mean, why pay Amazon big bucks for e-books when you can often find better books for free?)
This is the result of that huge Google scanning project that's been underway for years. I've downloaded books from Stanford and UC Berkeley's libraries, from Harvard's libraries (including the divinity school's library for you theologians), from the New York Public Library's research collection, and from many other similar places. Basically scanned versions of the entire holdings of out-of-copyright titles in all of those libraries is appearing here, for everyone to read for free.
Not only is much of the 19'th and early 20'th century scholarly literature here, you can even find titles from the 1700s and the 1600s. It's kind of fun to see what educated people were reading in those days and what the state of scholarship was in various fields.
It's kind of amazing and I like it.