Are you looking for information about books on the Internet? Do you need references of literature that will help you to solve a problem or complete a project? There's a lot of bibliographic information available on the World Wide Web. Generally speaking, there are three main sources: Libraries, Bookshops, and Publishers.
For literature on merely every topic the Library of Congress in Washington, D.C. (http://lcweb.loc.gov/homepage/lchp.html) is a wonderful source. They provide an easy-to-use form interface at URL http://lcweb.loc.gov/z3950/mums.html.Also many astronomy libraries offer their catalogs via the Internet, like the Space Telescope Science Institute library (http://sesame.stsci.edu/library.html, library catalog: telnet stlib.stsci.edu), the National Radio Astronomy Observatory (http://www.cv.nrao.edu/html/library/library.html, library catalog at http://www.aoc.nrao.edu/NRAOLIB.html), or the ESO libraries (http://www.eso.org/libraries/eso-libraries.html, library catalog at http://www.eso.org/libraries/webcat.html), to name just a few.
Specialized libraries will provide you with a wide range of (astronomy) books which you usually won't find on any bookstore's shelves. However, for general reference works bookshops on the Internet will do a good job. You might want to try Amazon.com Books (http://www.amazon.com), Book Stacks Unlimited (http://www.books.com), BookWire (http://www.bookwire.com) or the Internet Bookshop (http://www.bookshop.co.uk).Their online catalogs will allow you to search for literature covering a topic as well as books written by a particular author. In addition to the pure bibliographic details, you might also find prices and, depending on the bookshop, abstracts or reviews of book titles that will give you a more detailed idea of what the contents of the book really is about.
Finally, many publishers offer their catalogs on the WWW, although some of them are still under construction. In order to make full use of their service, you should be informed about the kind of titles they usually publish, otherwise you might end up searching publishers' sites that are irrelevant for your query. For the area of astronomy, try for instance Cambridge University Press (http://www.cup.cam.ac.uk/), Kluwer Academic Publishers (http://www.wkap.nl/) or Elsevier (http://www.elsevier.nl/) or Springer (http://www.springer.de or http://www.springer-ny.com).