A useful source for finding more books by an author you like (Amazon isn't always accurate!)
www.isfdb.org/
A similar site for Films (not just S.F.) www.imdb.com
Good general info site. www.sfsite.com/home.htm
One of the best Author specific sites, (Orson Scott Card). www.hatrack.com/A good fan site for Larry Niven. www.larryniven.org/
The website for the Sci-Fi channel (US version). www.scifi.com/home.html
One of my favourite Authors, not mentioned elsewhere! (Barrington J Bayley). oivas.com/bjb/
An excellent site for Star Trek Voyager. www.star-trek-voyager.net/index.htm
The Terry Pratchett website. The L Space Web
The Science Fiction Foundation. British organisation. www.liv.ac.uk/~asawyer/sffchome.html
Philip K Dick.
Voyager Books (nothing to do with the Trek Franchise!). Mostly paperback book published. http://www.voyager-books.co.uk/
Orbit Books. Only paperback books published. http://www.orbitbooks.net/
Another site for author bibliographies. www.fantasticfiction.co.uk/