Google is in the midst of an ambitious project to digitize books from publishers and in libraries. Those that were published before 1923 are in full text; those still potentially under copyright can be searched, but not viewed in full. It offers an interesting way to locate very specific words, phrases, and citations, particularly in older books. Using the advanced search you can limit a search to books that are full text or published within a range of years.
To request books or articles not available at K use the Interlibrary Loan System.