The vast majority of books are taller than they are high. It’s way more natural to read books on the iPad in portrait orientation, and that’s also how it’s shown in iPad commercials[1].
When you open a book (which is the only way you can read it!), it is definitely wider than it is tall. And hence I find it much more natural to hold a tablet in the landscape mode.
Most books have pages that are taller than they are high, the text blocks on those pages are definitely taller than they are wide. Paragraphs don’t run in one line from the verso to the recto[1]. A ‘page’ is one side of a sheet, you don’t read two pages at the same time.
[1] http://www.youtube.com/watch?v=bhC40QCZML0