CLIO is the library catalog for the Columbia Library System (including Barnard Library, but excluding Teachers College and some information from the Law Library). In CLIO you can find books and media materials, search for different kinds of databases, and find articles within those databases as well.
A very large index of books as well as magazines. If the book is out of copyright, or the publisher has given Google permission, you'll be able to see a preview of the book, and in some cases the entire text. If it's in the public domain, you're free to download a PDF copy.
If a title isn't available via Columbia University Libraries, you may request it via Borrow Direct or Interlibrary Loan (ILL). Try Borrow Direct first for books - the delivery faster and you can check books out for a longer period of time.