Google Books is a full-text searchable collection of millions of books. The books in Google Books come primarily from publishers and libraries (including libraries at Harvard, Oxford, the New York Public Library, the University of California, the University of Michigan, and Stanford).
Google Books provides varying levels of access to the full-text of each book: Full, Limited, Snippet, and No Preview. The "About Google Books" page explains more about each level of access. Even when the entire book isn't available online through Google Books, NYU Libraries may have it in print, or may even have access to an online copy.
Google Books is different from library catalogs like NYU Libraries Catalog or Worldcat, in that it lets you search inside the text of the book. Library catalogs like NYU Libraries Catalog and Worldcat are limited to searching only title, author, subject heading and other specific and limited information about a book.