Just to add to the previous message (Thanks Kimba15)
For people in US or where your local currency has a better exchange rate with USD than Pound -
www.bookdepository.com now exists too.
The free shipping is pretty much world wide. The delivery is reasonably fast for standard international delivery (from an Aussie perspective)- but obviously if you need something in a hurry you'll have to use a supplier with expedited delivery. Also I'm not sure how it compares with ordering from Amazon using standard shipping if you live in the USA. The only delay I had was when Europe was covered in volcanic ash and all planes were grounded
For Aussies - if you know what you want - try the following search engine which trawls through online book stores and calculates in Aussie Dollars the cost of the book including postage and handling:
www.booko.com.au They get a small commission if you click on their link and you purchase the book.