I'd like to take a moment to do something I don't usually do which is promote someone else. I would like to promote http://www.goodreads.com . This website is a great website to keep track of the books you're reading, books you've read, as well as finding new books you might be interested in. What's more this site gives you links to buy books you have been interested in and just haven't been able to find anywhere or anywhere for a cheap price.
You can write reviews on books you've read, invite you're friends to join and share books you think they may like. Being an avid reader I find this to be an invaluable source.
The best part about this site is its easy to join.
p.s. If you are a Science Fiction/Fantasy lover like me but are as picky about your books as I am I recommend the readers group: Beyond Reality.