Indeed,
www.fictionwise.com and
www.booksonboard.com are my two primary sources for ebooks, and you do save a bundle as the prices are cheaper then UK prices.
Fictionwise and Booksonboard can both accept payment from Uk credit card or through Paypal (My preferred method). BoB can also accept payment trough Google Checkout and they offer an addition 2% discount which you may as well take as it just takes a minute to setup the first time.
As for which store is cheaper, it really depends. If you are a fictionwise club member (Which costs $25 but you can often get deals to get it back as micropay) many books may work out cheaper. If you buy regularly, you can usually get back the savings in a few months.
Books on Board usually has prices cheaper then Fictionwise's Listed prices, but equal to or higher then the club prices.
But some books seem to regularly be cheaper at one or the other so its well worth checking on both.
www.Cyberread.com while not usually cheaper does currently have a discount code (THKU08) which will give you 50% discount on the books, so may be worth checking out.
Best of luck and welcome to the eBook Readers club
