At the iPad pages of Apple’s website, they are saying a little more about how their iBook venture will work and how it will be spelled: iBookstore!
Straight from Apple:
The iBooks app is a great, new way to read and buy books. Just download the app for free from the App Store, and you’ll be able to buy everything from classics to bestsellers from the built-in iBookstore. Once you’ve bought a book, it’s displayed on your Bookshelf. To read it, all you have to do is tap on it and it opens up. The high-resolution, LED-backlit screen displays everything in sharp, rich, color, so it’s very easy to read, even in low light.