I signed up for this site a few months back and I'm really enjoying it. They are known as "the Netflix of books" - $10 a month gets you unlimited access to their library of ebooks. It is a beautiful reading format for iPad and iPhone and they just added web browser reading also. I don't think it's available for Android yet, but I think they are working on it. I am always hesitant to sign up to any rebilling service but they offer a free trial month so I thought I would give it a shot for the month. Well it's been about 3 months now and I am still really loving it, I read a little bit on there every day and I think it's a pretty good deal, especially for people who buy a lot of ebooks.
You should check out Scribd.com as well then. Oyster and Scribd are both using the same model, and like you I love it! Only difference it sounds like is that the price for Scribd.com is $8.99 per month and not $10 per month. I'm loving the service so far though as well, and haven't had any books that I've looked up that I haven't found. My wife and I read a ton, so this could be one of the best buys of the year.