I've been reading a couple (three actually) books on the iPad and now the iPhone. iBooks is a great application! The only shortcoming is the availability of books - which should improve over time. I have other books in the Kindle app (also available for iPad and iPhone), but if a book is available for both platforms, I opt for the iBooks version.
iBooks is an excellent e-reader that feels impressively like the future—while simultaneously feeling a heck of a lot like an actual book. Right now, Amazon’s dominating selection renders it the top dog in the iOS e-reading world. But if the book you’re after is available for iBooks at a good price, the app provides an immersive and thoroughly pleasant reading experience, particularly on the iPad.