1- OCLC Online Union Catalog

You can find any book and the libraries that hold a copy of it.

2-  The library of Congress catalog

One of the best online catalog.

     3-  The British Library

This is the home page of the British Library website. Use this site to search our catalogues, order items for research, view exhibitions and link to 

     4-  Libdex :

            You can discover the directory of library across the world as well as

            an extensive  collection of books.

    5- Full-Text Online Library

         Over 76,000 full-text books & 2 million articles.

        6-  www.Questia.com/Online_Library

            Questia is the first online library that provides 24/7 access to

            the world's largest online collection of books and  journal

          articles in the humanities and social sciences, plus