At iMedicalApps we're huge fans of Inkling. Initially they launched only an iPad app, but they now have an iPhone app and you can also access their texts via a web browser. We recently did a review of their iPhone app that you can access here.
I'm a huge fan of inkling. I think they really make texts a lot more accessible. Great to test yourself on their books as well. I haven't used the social side of things much but if I have a choice then I will try use the inkling text version.
I know Inkling books aren't cheap, but as I live outside US, Inkling books are way cheaper than book price + shipping 90% of the times. Also, I like the interface and I prefer ebooks over physical books.