Marco Arment:

The books available on the iBookstore are just called books.

I had no clue, but this seems more clumsy.

With this method (as I read it) this statement would not be correct: “I own the Steve Jobs iBook.” Instead it seems the correct statement would be: “I own the Steve Jobs book, from the iBookstore.” That’s far more clumsy in my mind.

You could, technically, leave out the “from the iBookstore” bit, but in doing so you would have no way of indicating that you purchased a digital version instead of a dead tree version.