The Google Books project involved spending $300 million to scan 10 million books and make them available on the Web. This immediately caused legal strife with authors and publishers, but a settlement has been reached (at least in the USA). Robert Darnton in a NY Review article gives one perspective on the settlement, Lawrence Lessig in the New Republic another.
Legal issues aside, Google Books is enormously useful, especially if you study work in the public domain: up to the 19th and early 20th century. The fully searchable texts of numerous books are easily available, and they’re actual digitised text, not just scans of the pages. This enables you to do things a library simply can’t—for example, a student had a query about a law essay they were writing, which included a quote from someone called Bagehot about the “appendages of monarchy”. Searching for “appendages of monarchy bagehot” in Google Books takes you to page 315 of The English Constitution, which you can freely read online.
Not just read—you can quote it too. The toolbar in page view has a Plain Text option, where you can switch from scanned text to plain text, ready for copying and pasting.You can download pages, as PDFs or in the EPub format for e-book readers. And there’s also a Clip button, which lets you drag over the page to create, in effect, a screenshot. Google Books gives you an web address for that particular screenshot, and even a link you can paste into your blog that will display the snippet. Here’s a New Zealand reference from Darwin’s book The Formation of Vegetable Mould by the Actions of Worms: